短语讲解：

1. This is because of caffeine, a substance found in some plants.

because of 因为，由于
例句：I'm here because of you.
我来这儿是因为你们。

2. He and the other researchers worked with a group of 160 people who were caffeine free.

work with 与…在一起干; 从事…工作
例句：I work with disabled athletes.
我的工作就是帮助残疾人运动员。

3. On the second day, all of the subjects were asked to look at more pictures.

look at 看; 评判
例句：Look at our verse today.
看我们今天的章节。

4. And a third group of pictures was similar to the first group but a little different.

be similar to 与…相似; 类乎
例句：This condition could be similar to human depression.
这一状况类似于人类的低落情绪。

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