No matter how hard I tried to explain how to operate the machine, they were still at a loss.
A．尽管我努力把机器开动了，他们还是觉得非常失望。
B．无论我怎么努力地说明机器的用法，他们都不理解我。
C．即使我努力地对机器做了解释，他们还是不相信我的话。
D．不管我怎么努力地解释如何操作这台机器，他们依然听不懂。

A. buses are always available in 24 hours
B. bus riders have to buy monthly passes
C. bus fare is paid by exact change in coins
D. buses are the best means of transportation