问题补充：
--- Doctor, should I take this medicine? --- Twice a day.
A. how often
B. how much
C. how soon
D. what time
答案：
(答案→)A. how often
B. how much
C. how soon
D. what time
解析：句意：——医生，我应该多久吃一次药？—— 一天两次。A. how often 多久一次（就频率来提问）；B. how much 多少（就价钱来提问或者修饰不可数名词）；C. how soon 还要多久（常用在一般将来时态的句子中，其答语通常是“in + 一段时间”）；D. what time几点，根据一天两次，可知就频率来提问，故选择A。
