I don't know how( ) to London .By air

I don't know how( ) to London .By air
I don't know how( ) to London .——By air
A.would they go B.are they going C.they would go D.they are going
偶知道 但是能不能帮我分析一下C和D啊
有什么区别啊 我会非常感谢的。