^_ Who's that boy reading in the garden? Is it David?

^_ It __________ be David. I saw him in the classroom just now.[ ]

A. must

B. can't

C. may

D. shouldn't

自身免疫性胃炎患者出现恶性贫血是由于血中有抗内因子抗体存在从而影响（）

A.维生素C的吸收

B.维生素C的降解

C.维生素B₁₂的吸收

D.维生素B₁₂的降解

E.维生素K的吸收