— Is Jack on duty today?

— It ____ be him; it's his turn tomorrow. [ ]

A. must's

B. won't

C. can't

D. needn't

乾隆、嘉庆时期的学者奉顾炎武为考据学的开山大师，道光、咸丰时期的学者则推崇他的“经世致用”之学。这一学风的变化折射出（）

A.儒家思想正统地位的削弱

B.政府文化专制政策的松动

C.科举考试内容发生了巨变

D.清朝后期社会危机的加剧