By the time Neil Armstrong and Edwin Aldrin walked on the moon i

By the time Neil Armstrong and Edwin Aldrin walked on the moon in 1969, there ______ many space explorations.
A、will have been
B、had been
C、have been
D、must be
【正确答案】：B
【题目解析】：本题考察完成时态, By the time是运用完成时的时间状语标志之一，由于是By the time...in 1969, 是过去发生的事情了，所以用过去完成时，B项符合题意。