We must take two aspects into consideration, both of __________ associated with the safety of the students .

A．which

B．whom

C．that

D．them

自病原体侵入机体到临床症状最早出现的一段时间称为（）。

A.潜伏期

B.传染期

C.前驱期

D.发病前期

E.隔离期