My brother_____while he_______ his bike and hurt himself.
A:fell; was riding
B:fell; were riding
C:had fallen; rode
D:had fallen; was riding
某校创办了“创客社团”,该社团的同学开展了如下四个探究活动:
下列各项,属于影响人口分布决定性因素的是( )
估计31-2的值( )