阅读理解,回答下列问题
It is Sunday morning. Helen and David are going to the People Cinema by bike. They are coming up to a crossing.
Helen: Stop, David!
David: Why? It's late. We should hurry up.
Helen: Look at the lights. It's red now, and it means “Danger”. The cars and the buses must wait, and we must stop too.
David: What does the yellow light mean?
Helen: It means “Wait”. We should wait till(直到) the light is green.
David: Yes, I see. The light is green now. We can go now.
Helen: OK! Let's go.