F344/N与（）小鼠相同，均可作为视网膜退化的模型。
A、CBA/J
B、NZB
C、A/J
D、DBA/2
参考答案：A
已知点P在∠AOB内，过点P作PE∥OA，PF∥OB，则∠EPF与∠AOB有何关系？（用量角器测量后得出结论）
— Is Jack on duty today?
— It ________ be him. It's his turn tomorrow.
A．mustn't
B．won't
C．can't
D．needn't
