He wrote many children's books, nearly half of____ were published in the 1990s.
A:whom
B:which
C:them
D:That
一列简谐波向右传播,沿波的传播方向上有相隔一定距离的P、Q两点,当P点经过平衡位置时,Q点也恰好经过平衡位置,如图所示,已知P、Q间只有一个波峰,试画出P、Q间可能的波形。
如图,D、E分别是△ABC的边AB、AC上的点,且AD=DB,AE=EC,若DE=4,则BC长为( )
设c,b是两条直线,α,β是两个平面,下列能推出c⊥b的是( )