补充250-96

There is substantial evidence that certain forms of solar energy either now or within a few years will be economically competitive with conventional sources of heat and power.

(A) either now or within a few years will be economically competitive with conventional sources of heat and power

(B) will either be economically competitive with conventional sources of heat and power within a few years or are so now

(C) will be economically competitive with conventional sources of heat and power either now or within a few years

(D) either are now economically competitive with conventional sources of heat and power or will be so within a few years

(E) are either now or will be within a few years economically competitive with conventional sources of heat and power

答案为D，我怎么觉得C读的更顺口阿

还有，大全的详细解释在哪儿可以下载阿

got it~~~

thank you