958. 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（D）
(E) are either now or will be within a few years economically competitive with conventional sources of heat and power
这道题目either, or是修饰时间的，为什么不能选c呢？
狂不明白 求教
either now or within a few years表示现在或者将来，因此要配合两个不同的时态，一般现在和将来，A/C均只使用了将来时，有问题排除。这体现了GMAT对句子逻辑严谨的要求。
另外提供你一个方法：看到now/a few years表示时间，通过句子意思理解是做时间状语来修饰谓语，一定要注意它们的放置位置是有严格要求的。通常不能放在助动词之前，而需要放在助动词之后或表意思动词之后，通过这个就可以把A/E排除，然后利用平行原则，把B/C排除，选D
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