A、By air. B、By bus. C、By train. A根据问题“格林一家什么时候坐飞机去伦敦了?”可知正确答案为A。

W: What’s the time?
M: Ten past eight.
W: Oh, we have to hurry. The meeting will being in half an hour.
Q: When will the meeting begin?

W: When did the Greens fly to London?
M: Three hours ago.
Q: How did the Greens go to London three hours ago?