机体抗感染的第一道防线是（）
A.屏障结构
B.吞噬细胞
C.体液免疫
D.细胞免疫
E.NK细胞
参考答案：A
England has many rivers, _____ are very long and wide. [ ]
A. none of which
B. all of them
C. every one of them
D. neither of which
离心泵的轴承箱用30#机油或40#机油代用；蒸汽往复泵汽缸用（）油。
