Space exploration began in the 1960s, and since then about five hundred peopleinto space.

发布时间：2020-08-16 22:20:40

Space exploration began in the 1960s, and since then about five hundred people    into space.
A. have traveled
B. traveled
C. had traveled
D. are traveling

网友回答

(答案→)A. have traveled
B. traveled
C. had traveled
D. are traveling
解析：本题考查时态。根据时间状语since then可知与完成时连用。句意：太空探索开始于20世纪60年代，自从那以后大约有500人游览过太空。
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