In general,a typical （）model is composed ofseveral phases, such as requirements analysis phase, general/detailed design phase, implementation phase, system acceptancetesting phase.

A．waterfall

B．incremental

C．spiral

D．prototyping

缺铁性贫血最常见的病因是

A．慢性胃炎

B．慢性肝炎

C．慢性溶血

D．慢性感染

E．慢性失血