A:3∶2∶2
B:1∶1∶1
C:3∶1∶1
D:1∶3∶3
计算:
He ____ been afraid of dogs ____ ____ since his dad kept a pet dog.
“几处早莺争暖树,谁家新燕啄春泥”,这句诗描述的是鸟类繁殖行为中的( )
下列有关实验操作叙述正确的是( )