— Listen! Someone is knocking at the door. Who can it be?
— It _______ be Tom. He said he would visit me this evening.
A:can't
B:need
C:must
D:mustn't
目前我国人口增长迅速,将会带来的后果是( )
①消费群体扩大,促进市场经济发展 ②劳动力充足,促进生产发展
③资源需求膨胀,人均资源量少 ④阻碍经济发展,不利于人民生活水平的提高
函数y=x-4中,自变量x的取值范围是( )