Because it has something covering it and he has to watch his back
The correct option is A.
From the passage given above, it can be seen that the author used the illustration of green house and its mechanism to show us the way in which the atmosphere works. The author wants us to see that just the way the glass capture the heat of sunlight and prevent it from escaping, the atmosphere works in the same manner by trapping the heat on earth preventing it from escaping, thus keeping the earth warm.<span />
It's a metaphor.
To help you understand why, here's the definitions of both:
Simile: it's a <span>figure of speech involving the comparison of one thing with another thing of a different kind, it's used to make writing seem more vivid.
Metaphor: this is also a</span><span> figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable.
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