When I passed this afternoon, I saw a little boy was lying on the grass, his hands _____ under his head.
A：having crossed
B：crossing
C：crossed
D：to cross
判断对错
4米的 15 和1米的 45 一样长.
①孟德尔的豌豆杂交实验提出了基因的分离定律和自由组合定律
②萨顿发现基因和染色体的行为存在明显的平行关系，提出假说“基因在染色体上”
③摩尔根进行果蝇的杂交实验，证明基因位于染色体上
