A:that
B:who
C:what
D:which
Listen, the boys ______ with each other over there. Please go and see what has happened.
“蓝蓝的天上白云飘,白云下面马儿跑.挥动着鞭儿响四方,百鸟齐歌唱.”这首歌描述的乡村聚落类型是( )
地球上适合生物生存的地方,科学家称其为____.它包括____的底部、____ 的大部和____的表面.其厚度约为20千米.