Space exploration began in the 1960s, an
题型:选择题
问题:
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
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