单选题 . ①Hotter and more massive than the Sun, stars called "stragglers" are puzzling to astronomers because such rapidly burning stars would not be expected to persist in ancient star clusters. ②Some researchers believe that the typical blue stragglers formed when two ancient, lower-mass stars collide and merge form more massive, hotter star. ③Peter Leonard theorizes alternatively that in low density globular clusters, where mergers between single stars occur too infrequently to account for the observed quantity of blue stragglers, these stragglers are created instead by a group of stars. ④He suggests that a pair of stars already orbiting each other presents a larger target for a third star or another pair. ⑤Once this new grouping forms, close encounters between the stars could prompt any two to merge as a blue straggler. ⑥Leonard's model predicts that each blue straggler has a distant orbiting companion—as appears true of many blue stragglers in the M67 cluster of the Milky Way galaxy.

21. The reference to a "larger target" serves primarily to suggest why a ______

A、 blue straggler would be more likely to collide and merge with another star than would be a lower-mass star
B、 pair of stars would be more likely to encounter other stars than would the typical blue straggler
C、 pair of stars would be more likely to interact with other stars than would a single star
D、 blue straggler would be more likely to interact with a pair of stars than it would with a third star
E、 third star would be more likely to encounter a pair than it would to encounter a blue straggler

【正确答案】 C

【答案解析】 C：“更大的目标”就是为了说明成对的恒星比单颗恒星更有可能发生撞击，因此选项C正确。
A、B、D、E：根据原文，蓝色离散星是撞击之后的结果，选项A、B、D、E全都在说蓝色离散星会其他恒星发生碰撞，因此错误。