问题
选择题
Space exploration began in the 1960s, and since then about five hundred people _______ into space. [ ]
A. are traveling
B. traveled
C. had traveled
D. have traveled
答案
答案：D
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Space exploration began in the 1960s, and since then about five hundred people _______ into space. [ ]
A. are traveling
B. traveled
C. had traveled
D. have traveled
答案：D