练习材料：
[Day 1750 2019-07-12]
Lesson 20-1 Snake poison
How it came about that snakes manufactured poison is a mystery. Over the periods
their saliva, a mild, digestive juice like our own, was converted into a poison that defies
analysis even today. It was not forced upon them by the survival competition; they
could have caught and lived on prey without using poison, just as the thousands of nonpoisonous snakes still do. Poison to a snake is merely a luxury; it enables it to get its
food with very little effort, no more effort than one bite. And why only snakes? Cats,
for instance, would be greatly helped; no running fights with large, fierce rats or tussles
with grown rabbits -- just a bite and no more effort needed.
任务配置：L0+L1+L4
知识笔记：
saliva n. 唾液;
defies v. 违抗; 反抗; 蔑视; 不可能，无法(相信、解释、描绘等); 经受住; 顶住; 抗住;
analysis n. (对事物的) 分析，分析结果; (对物质的) 分析;
prey n. 猎物; 受害者; 受骗者; v. 捕食; 掠夺; 折磨;
nonpoisonous 网络 无毒的; 无毒; 无毒性的;
tussles n. 扭打，争斗，争执(尤指为了争得物品); v. 扭打，争斗(尤指为了争夺物品);
grown adj. 成熟的; 成年的; 长大的; v. 扩大; 增加; 增强; 长大; 长高; 发育; 成长; (使) 生长;
练习感悟：
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