— _______________ the pen?
— It's in your pencil-case. [ ]
A. Where
B. What's
C. Where's
答案:C
汗液的排泄通过()
A.腺体暗细胞的收缩
B.腺体透明细胞的收缩
C.肌上皮细胞的收缩
D.导管壁细胞的收缩E导管口细胞的收缩
润滑有何作用?试述典型压力润滑系统的流动路线。