因为这个概念不能直接得出是通过推到的出的concept所以教授说这是一个
difficult point ，
前面的句子是
We call all of this the land phase of the Phosphorus cycle. But a lot of the phosphorus in the soil gets washed away into rivers by rain and melting snow. And so begins another phase of the cycle. Can anyone guess what it’s called? Nancy?
综合考虑 struggling可以看出来吧！
你选c正好相反。
