In today’s fast changing and competitive world, to be able to learn faster and more effectively is not merely desirable; it is ________.

A．essential

B．worthwhile

C．challenging

D．sufficient

分布于小气道内的咳嗽感受器为（）

A.机械感受器

B.化学感受器

C.神经感受器

D.压力感受器

E.其他感受器