Are you going to going to Beijing by plane?

Are you going to going to Beijing by plane?
It's fast, but expensive. so I am not sure. I _____ take a train.
a. should b. may c. must d. will
请问这个题选择哪个呀?为什么呀?
我选择的是B可是答案说是C 啊?到底怎么做呀?谢谢啦嘿嘿.
重要的是说一下为什么呀？谢谢啦嘿嘿。。。