Whenever we kids come over, Auntie Susan just stands there and watches us ________ sure we don't break anything.
A:make
B:made
C:to make
D:making
几位同学用一个弹簧测力计来比试臂力,如图所示,大家都不甘示弱, 结果每个人都能把手臂撑直,则( )
北半球夏季长于冬季是由于( )