请教PREP 133

133.       (31602-!-item-!-188;#058&004586)

In the past several years, astronomers have detected more than 80 massive planets, most of them as large or larger than Jupiter, which circle other stars.

(A) most of them as large or larger than Jupiter, which circle

(B) most of them as large or larger than Jupiter and circling

(C) most of them at least as large as Jupiter, circling

(D) mostly at least as large as Jupiter, which circle

(E) mostly as large or larger than Jupiter, circling

这道题目答案是C，我觉得ABC选项中的them有指代歧义的嫌疑，既可以指代astronomers因为是主语，也可以指代planets因为就近，为什么选呢？解释说mostly是副词不对，为什么呢？