16.维生素A与三氯化锑的呈色反应，其生成物的颜色为

A.蓝色                                                          B.紫红色

C.蓝色渐变成紫红色                                     D.黄色

17.在甾体骨架中，环戊烷属于

A.A环                                                           B.B环

C.C环                                                           D.D环

18.可用硫色素反应碱性鉴别的药物是

A.维生素A                                                    B.维生素B1

C.维生素C                                                    D.维生素D

19.鉴别黄体酮灵敏，专属而又简便的试剂是

A.间二硝基苯                                                B.异烟肼

C.亚硝基铁氰化钠                                         D.硝酸银

20.青霉素分子中最不稳定的结构为

A.酰胺基                                                       B.β-内酰胺环

C.羧基                                                          D.苄基