A good storyteller must be able to hold his listeners' curiosity _____ he reaches the end of the story. [ ]

A. when

B. unless

C. after

D. until

当某种卫生服务价格变动对需求量变动无影响时，则该服务为（）。

A.完全弹性

B.富有弹性

C.单元弹性

D.缺乏弹性

E.完全无弹性