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