Peter _____ be really difficult at times even though he's a nice person in general. [ ]

A. shall

B. should

C. can

D. must

感染转为慢性结局是由于（）

A.机体抵抗力与细菌毒力处于相持状态

B.治疗不当

C.人体抵抗力下降

D.细菌毒力增大

E.细菌变异