—______ he from the U.S.A.?
—No, he’s from China.[ ]
A. Are
B. Is
C. Am
答案:B
对流免疫电泳中抗原和抗体带的电荷分别是()
A.+,-
B.+,+
C.-,+
D.-,-
E.不带电荷
企业有哪些任务?