题目

Dear Sir,       I have to travel every day from Souk Road to the airport. Two buses travel along this route (路线): the number 49 and number 16. But by the time the number 16 bus reaches Souk Road it is always full, so I can’t get on it. I have to wait for the number 49, because sometimes there are empty seats on it.      The timetable says that there are buses from Souk Road to the airport every ten minutes. If this is so, why do I have to wait half an hour for a bus nearly every day? The regulations (规章) say that if there are empty seats on a bus, the bus must stop –Could I make a few remarks on the discussion?  --Yes, of course you _________. A. should      B. must       C. can       D. could