问题
选择题
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
答案
答案：A
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。