问题:
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
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
A.have traveled
B.traveled
C.had traveled
D.are traveling
参考答案: