早期妊娠时，羊膜囊被下列哪一结构围绕（）
A.羊膜
B.绒毛膜
C.蜕膜
D.胎盘
E.卵黄囊
参考答案：B
It's wrong ______ not ______ her an e-mail. [ ]
A. for you, post
B. for me, reading
C. of him, sends
D. of you, to send
能与MHC-Ⅱ类分子相结合的分子是
A．CD3B．CD4C．CD8D．CD16E．CTLA-4
