C ．荚膜                                                                 D ．外毒素

19 ．水、饮料、食品的卫生学检查中常作为被粪便污染指标的是细菌总数和 (       )

A ．大肠杆菌菌群指数                                            B ．痢疾杆菌菌群指数

C ．变形杆菌菌群指数                                            D ．伤寒杆菌菌群指数

20 ．汉坦病毒的主要宿主是 (       )

A ．蚊                                                                     B ．鼠

C ．蜱                                                                     D ．虱

21 ． HIV 侵入机体后能选择性地感染 (       )

A ．  T 细胞                                                    B ． 细胞

C ．  T 细胞                                                    D