将轻绳和轻弹簧的一端分别固定在圆弧上的A、B两点,另一端固定在小球a上,静止时,小球a恰好处于圆心O处,如图甲所示,此时绳与水平方向夹角为30°,弹簧恰好水平,现将轻弹簧与轻绳对调,将a球换成b球后,小球仍位于O点,如图乙所示则,a、b两个小球的质量之比为( ) A.1∶1 B.∶1 C.2∶ D.3∶2
We experienced a few moments as we waited to hear the results of the test.( )
A.pleasantB.anxious
C.hopefu1D.disappointing
What are you doing?
I'm looking ______ the kids. They should be back for lunch now.( )
A.afterB.atC.forD.up
Do you still play basketball?
Oh, no. I ______ it for the past three years.( )
A.haven't playedB.didn't play
C.won't playD.hadn't played
I still don' t know what____ while I was away from home.
You'll know it all some day, l believe.( )
A.had happenedB.would happen
C.has happenedD.happened
My sister______ the piano very well, but she hasn't had time to play recently.( )
A.will playB.has playedC.playedD.plays