A:at that time
B:by that time
C:at which time
D:by which time
在直角坐标系xOy中,直线l:y=t(t≠0)交y轴于点M,交抛物线C:y2=2px(p>0)于点P,M关于点P的对称点为N,连结ON并延长交C于点H.
My mother __________ at the school gate now.
关于地球公转的说法,正确的是( )