下列分子和离子中中心原子价层电子对几何构型为四面体且分子或离子空间构型为V形的是 [ ]
A.NH4+
B.PH3
C.H3O+
D.OF2
答案:D
—Can I go to the park, Mum?
—Certainly. But you ______ be back by six o’clock.
A.can
B.may
C.might
D.must
B细胞能识别特异性抗原,因其表面有()
A.Fc受体
B.C3受体
C.IPS受体
D.E受体
E.SmIg