下列单项式-x,2x2,-3x3,4x4,…中,第n项是( )。
(-1)n·n·xn
王小明同学使用显微镜时,老师告诉他:视野太暗,老师轻轻转动了反光镜视野就变亮了.老师这时选择的是( )
A.凹透镜
B.凹面镜
C.平面镜
D.凸面镜
He is a man of few words and it is hard to ________ his thoughts.
A.see
B.tell
C.read
D.say