A：are
B：i
C：have
D：ha
坐下之前他们握了手。
They____ ____before they sat down．
下面四幅地形剖面示意图中，最能反映我国地势特征的是（ ）
When the head teacher was walking past the window, she noticed two students _______ chess in class. She really got _______.
关于超声波和电磁波，下列说法中正确的是（ ）
