Could you say it again? I can't understand _________ you are talking about.
A：how
B：when
C：what
D：which
— How much does this camera _______, please?
— Four hundred dollars.
小狗建立的“铃声﹣﹣唾液分泌”反射的类型和完成该反射的神经结构分别是（ ）
在图中，A、B两球相互间一定有弹力作用的图是（ ）
