181. In some species of cricket, the number of chirps per minute used by the male for attracting females rise and fall in accordance with the surrounding temperature, and they can in fact serve as an approximate thermometer.

A. for attracting females rise and fall in accordance with the surrounding temperature, and they can in fact serve.

B. for attracting females rises and falls in accordance with the surrounding temperature, which can in fact serve

C. in attracting females rise and fall in accordance with the surrounding temperature, in fact possibly serving.

D. to attract females rises and falls in accordance with the surrounding temperature, and it can in fact serve.

E. to attract females rises and falls in accordance with the surrounding temperature, in fact possibly serving.

Answer: D

我选E，前面一整段话对后面的影响用V+ING最好，而且我觉得D中it指代有问题，希望有高人能够解释一下，谢谢！

 it指代number,因为serve as an approximate thermometer(温度计),数字随环境上下浮动,就象个温度计一样.情态动词can不能省略，E不论说它是指代整个句子，还是伴随状态，都是有问题的,改变句意不说，上下浮动这件事不能象温度计，而是数字象温度计，因为它随环境上下波动

请指教