A:①②
B:①③
C:①④
D:②④
Parents always tell us not to stay ________ there is danger.
人踢球时,对球施力的物体是____ ,同时____ 也受到球的作用力,这一事例说明____ .
It's almost eight years we _______ each other last time.
设a∈R,则“a=1”是“直线l1;ax+2y=0与直线l2;x+a+1y+4=0平行的( )