A:受精卵
B:生殖细胞
C:精子
D:卵细胞
—I think my Maths teacher is working hard. He teaches us very .
—Yes, but he can't come today. He doesn't feel .
近年来,发达国家出现“逆城市化”现象,城市人口向郊区、乡村居民点和小城镇流动,其原因并不是由于( )