He _____ have finished his work; otherwise, he wouldn’t be enjoying himself by the seaside.

A．should

B．must

C．need

D．can

低氧对呼吸的刺激作用是通过 ( )

A．直接兴奋延髓吸气神经元
B．直接兴奋脑桥调整中枢
C．外周化学感受器所实现的反射性效应
D．刺激中枢化学感受器而兴奋呼吸中枢
E．直接刺激呼吸中枢