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.