某个体基因型为Aa，让其连续自交三代，则后代中基因a的频率为[ ]
A．12.5%
B．25%
C．50%
D．无法估算
答案：C
--- Tom, why didn’t you come to the meeting yesterday?
--- I _____, but my car broke down.
A．would
B．had
C．was going to
D．did
