葡萄胎来源于何种细胞()
A．蜕膜细胞
B．胎盘内血管内皮细胞
C．子宫内膜细胞
D．滋养层细胞
E．原始生殖细胞
参考答案：D
Better go and ask him when he _____. We must see him off when he _____. [ ]
A. is leaving; leaves
B. leaves; is leaving
C. leaves; left
D. left; was leaving
