对下边漫画《孝心》理解正确的是[ ]
A.这是子女对母亲抚育之恩的回报
B.父母有抚养子女的义务
C.子女没有以实际行为孝敬父母
D.这是子女在精神上给予父母的慰藉
答案:C
_____earth scientists understand how and why earthquakes happen, forecasts are usually inaccurate.[ ]
A. As
B. While
C. Because
D. If
如图所示,用F =150N的水平力把重50N的物体压在竖直墙壁上,当物体静止时受到摩擦力的大小为
A.0N
B.50N
C.150N
D.200N