如图所示,放在M、N两水平桌面上的P、Q两物体,分别在FP=5N、FQ=3N的水平拉力作用下做匀速直线运动,可以确定( )
-- Don't touch the papers.
-- ____ (certain), I'll not let the papers be touched.
When I looked out of the window, I ____ smoke.