A. Let me see. I think it's a toilet for a pet（宠物）.

B. Yes. I do. I like studying them. 

C. Do you know anything about the public signs?

D. OK. What does that sign on the door mean?

一个球的质量为m,用绳系着放置在倾角为θ的光滑斜面上,细绳处于竖直方向上,如图所示,则斜面对球的支持力为(       )

下列含有细胞核的是（    ）

用如图甲所示装置研究水沸腾时温度变化特点，水的温度随时间变化如图乙．