It was snowing hard_______we had to stay at home and watch TV.
A:that
B:so
C:but
D:because
下列三张图所揭示出的我国古代商业活动中心发展的趋势是( )
如图是人体部分生理活动示意图,请据图回答:
下列关于人体呼吸的叙述,正确的是( )