第一段
The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, but this is largely because, unlike animals, we stand upright. This means that our noses are limited to perceiving those smells which float through the air, missing the majority of smells which stick to surfaces. In fact, though, we are extremely sensitive to smells, even if we do not generally realize it. Our noses are capable of 7 human smells even when these are diluted to far below one part in one million.
人类鼻子的功用没有得到应有的认识。人类通常被认为嗅觉不如动物灵敏，但这主要是因为，和动物不同，我们是直立的。这就意味着我们的鼻子仅能感知在空气中飘过的气味，而闻不到大多数附着于地面的气味。可实际上我们对气味极其敏感，尽管我们通常并未意识到这一点。即使把人类气味的浓度稀释到远不及原来的百万分之一，我们的鼻子仍然能够察觉到它们的存在。
01词海拾珠
·underrated 美[ˌʌndə'reɪtɪd]
vt.看清，低估；adj.低估的，被过低评价的
·insensitive/sensitive一对反义词 [be ~ to]
sensitive 美 [ˈsɛnsətɪv]
adj.敏感的；灵敏的；敏锐的；保密的；限制透露的；脆弱 的；感光的；易激怒的 n.具有超自然通灵能力的人
·upright 美 /ˈʌpraɪt/
adj. 正直的，诚实的；垂直的，直立的；笔直的；合乎正道的 n. 垂直；竖立
·perceiving
perceive 美 [pərˈsiv] v.认识到；意识到；理解；把…看作；认为；感觉；察觉；看出
·float
v. 使漂浮，浮动；漂流，飘动；飘移；安排（贷款）提出，提请考虑（想法或计划）；发行（股票）上市；（货币汇率）自由浮动实行
·dilute 美 /daɪˈluːt/
v. 稀释，冲淡；削弱；变稀薄，变淡 adj. 稀释的；淡的；（颜色、光线）浅的
·part n.（质量单位的）等份，份【熟词僻义】
·be compared with 与……比较
·be limited to VS be confined to
都能表示空间、面积、时间、活动等方面的限制，但be limited to还可表现能力局限
·stick to 坚持；粘住
【近义词】：stand on；persist in
·even if VS even though
even if 引导的从句往往是假设性的，“即使、纵然、就算、哪怕”；even though 引导的从句往往是真实的，“尽管、虽然”
·if only VS only if
if only表示一种虚拟语气，“要是……就好了，但愿”；only if “只有在……时候”
【例句】If only he comes early. 但愿他早点回来。
I wake up only if the alarm clock rings. 只有闹钟响了，我才会醒。
·be capable of 能够；有能力；有做出倾向的；有能力干
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02 语法释疑
This means that （that 引导的宾语从句，作means的宾语）our noses are limited to perceiving those smells which（which 引导的定语从句，修饰 smells） float through the air, missing the majority of smells （现在分词短语作伴随状语，miss与 our noses 逻辑上的主谓关系，所以用现在分词）which（which 引导的定语从句，修饰 smells）stick to surfaces.
第二段
Strangely, some people find that they can smell one type of flower but not another, whereas others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate particular smell receptors in the nose. These receptors are the cells which sense smells and send messages to the brain. However, it has been found that even people insensitive to a certain smell at first can suddenly become sensitive to it when exposed to it often enough.
奇怪的是，一些人发现他们可以闻到某一种花香，却闻不到另一种，而另一些人则对两种花香都很敏感。这可能是因为前者缺乏某种能够在鼻子中生成特定味觉感受器的基因。这些感受器是一些可以感知气味并向大脑发送信息的细胞。然而，研究发现，即使是开始对某种特定气味不敏感的人，如果经常接触这种气味，也会突然变得对它敏感起来。
01词海拾珠
·generate 美 /ˈdʒenəreɪt/
vt. 使形成；发生；生殖；产生物理反应
·receptors
n. [生化] 受体；接受器，感受器；神经末梢
·sense 美 /sens/
n. 感觉，官能；观念；道理；理智 vt. 感觉到；检测
【词组】make sense 有意义；讲得通
·be exposed to 接触；遭受；暴露；面对
02语法释疑
However, it has been found that（it为形式主语，that引导的主语从句做真正的主语）even people insensitive to a certain smell at first（形容词短语作后置定语，修饰people）can suddenly become can suddenly become sensitive to it when （when引导的时间状语从句）they are （文中省略they are，其中they 代指 people insensitive...，expose 和 they 为逻辑上的动宾关系，用被动）exposed to it often enough.
03写作佳句
·stangely
副词开头修饰全句，表示说话人对全句的看法，在意思上与“It is+adj+that...”相同。
常见副词有：luckily, incredibly, insterestingly, obviously, undoubtedly, apparently, surprisingly...
除此之外，还可以用插入语开头，表示说话人的态度。例如：
·Strangely enough 说来也奇怪
·Generally speaking 一般来说
·No wonder no doubt 毫无疑问
·In other words in my opinion 换句话说，在我看来
·In conclusion 最后
·In fact 事实上
·As a matter of fact 事实上
第三段
The explanation for insensitivity to smell seems to be that brain finds it inefficient to keep all smell receptors working all the time but can create new receptors if necessary. This may also explain why we are not usually sensitive to our own smells --- we simply do not need to be. We are not aware of the usual smell of our own house but we notice new smells when we visit someone else's. The brain finds it best to keep smell receptors available for unfamiliar and emergency signals such as the smell of smoke, which might indicate the danger of fire.
（人类鼻子）对气味不敏感的原因似乎是这样：大脑发现让所有的气味感受器一直工作效率太低，不过大脑可以在必要时产生新的感受器。这也可以解释为什么我们总是对自身气味不敏感——只不过是不需要而已。我们察觉不到自己家中惯有的气味，但是当我们拜访其他人的家时就会察觉到不熟悉的气味。大脑发现让气味感受器可随时用于接受不熟悉的、危急的信号，例如可能意味着有火灾的烟味，才是最佳选择。
01词海拾珠
·insensitivity 美 /ɪnˌsensəˈtɪvəti/
n. 感觉迟钝；不灵敏性；昏迷
·inefficient VS ineffective
inefficient 美[ˌɪnɪˈfɪʃnt] adj. 无效率的，效率低的；无能的
ineffective 美 /ˌɪnɪˈfektɪv/ adj. 无效的，失效的；不起作用的
·simply（引出概括或解释）不过，只是【熟词僻义】
【例句】I don't want to be rude, it is simply that we have to be careful who we give this information to. 我们不是有意无礼，只不过这份资料给谁我们必须很慎重。
·available 美 /əˈveɪləbl/【重要】
adj. 可获得的；可购得的；可找到的；有空的
·indicate 美 /ˈɪndɪkeɪt/
vt. 表明；指出；预示；象征
·be aware of 意识到；觉察到；知道
02语法释疑
·The explanation for insensitivity to smell （介词短语作后置定语，修饰explanation）seems to be that（that引导的表语从句）brain finds it（it作形式宾语，动词不定式to keep all...作真正的宾语）inefficient to keep all smell receptors working all the time but（连接并列谓宾, finds ...与can create...，表转折）can create new receptors if（if引导条件状语从句） necessary.
·The brain finds it（it作形式宾语，动词不定式作真正的宾语）best to keep smell receptors available for unfamiliar and emergency signals（形容词短语作smell receptors的补足语）such as the smell of smoke（举例说明 unfamiliar and emergency signals）, which（which引导的非限制性定语从句，修饰the smell of smoke）might indicate the danger of fire.
03写作佳句
此篇文章中我们主要积累一些表示原因的句型：
【第三段】The explanation for ... seems to be that ...
……的原因似乎是这样：
The explanation for ... is that ...
……的原因是这样：
除上述句型，相似句型还有The reason why ... is that ...
……的原因是这样：
【第三段】This may explain why ...
这也可以解释……
【第二段】This may be because ...
这可能是因为……
【第一段】This is largely because ...
这主要是因为……
其他说明原因的句型：
·A number of factors could account for ...
·There are some reasons/causes for ...
·The change in ... largely results from the fact that ...
·...（结果） lie in the fact that ...
·One of the most common factors（causes）is that ...
·Another contributing factor（cause）is...
·【强调结构】It is precisely because ...（原因） that ...（导致结果）
·句子开头表示原因的短语：Therefore, / As a result, / For this reason, / Consequently
·表示原因的短语：owe to / due to / thanks to / lead to / results in / results in / attribute to / be responsible for / account for / stem from
·fundamental cause 根本原因
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