流感病毒容易发生变异的原因是()
A.有两层包膜
B.表面分布有血凝素
C.表面分布有神经氨酸酶
D.核酸分节段
E.核心中含RNA多聚酶
参考答案:D
The successful launch of Shenzhou VII must have immediately excited the Chinese, _____ it? [ ]
A. needn't
B. didn't
C. hasn't
D. mustn't
简述汇付的含义及种类。