21.下列元素中屬于微量元素的是（      ）

A.鐵                                                            B.硫

C.鈉                                                            D.鎂

22.成年人缺碘會發(fā)生（      ）

A.克汀病                                                     B.甲狀腺腫

C.夜盲癥                                                     D.脊柱裂

23.異食癖患者主要是由于缺乏（      ）

A.鋅                                                            B.硒

C.鈣                                                            D.碘

24.引起人類患壞血病的原因是缺乏（      ）

A.維生素B₁                                                 B.維生素A

C.維生素C                                                  D.維生素D

25.缺乏維生素B₂的典型癥狀是（      ）

A.夜盲癥                                                     B.腳氣病

C.地圖舌                                                     D.壞血病