If you go by ______ train, you can have quite a comfortable journey, but make sure you get ______ fast one.
A.the; theB./; aC.the; aD./; /在线课程B
句意：如果你乘火车的话，你的旅途会很舒服，但你务必要乘快速列车。“by+交通工具”为固定用法。第二个空处选择不定冠词用来表示泛指，a fast one=a fast train。