下列可用于观察染色体结构的材料是( )
①洋葱鳞茎表皮 ②小麦的根尖 ③动物卵巢 ④鼠的神经元
A:①
B:①②
C:②③
D:②③④
“农田多水田,以种植水稻、油菜等为主,一年两至三熟”是对下列哪个地区农业情况的描述( )。
As soon as he _____ the bus , the poor man realized he had left his wallet on it.