1．As a young doctor, John Snow became frustrated because no one knew how to prevent or treat cholera.
2．One theory was that bad air caused the disease.
3．The other was that cholera was caused by an infection from germs in food or water.
4．The truth was that the water from the Broad Street pump had been infected by waste.
5．That's why we've given you the letter.
6．It looks as if it is going to rain.
[我的发现]
(1)句1中系动词became后接形容词作表语；句2、3、4、5、6中作表语的是从句。
(2)句2、3、4中的连接词that 在句中只起连接作用，没有含义，且不可省略。
(3)句5和句6中引导表语从句的连词有一定的含义，分别意为“为什么”和“好像”。
