-What were you doing at 8:00 last night ?
-Oh, I was busy _________ my homework
A:do
B:doing
C:did
D:done
下列关于癌细胞的叙述,错误的是( )
中国两名太空飞行员搭乘神州载人飞船进入中国空间站“天宫号二号”绕地球飞行,现已安全返回.飞船发射时用长征火箭发射.
下列有关某湖泊生态系统描述正确的是