单项选择题

M: Excuse me. Have you been waiting long
W: Twenty minutes already.
M: Did you notice whether the No. 7 bus has passed by
W: Not while I’ve been standing here. I’m also waiting for the No. 7 bus myself.
M: Good. Hot today, isn’t it
W: Yes, it is. I wish that it would rain and cool off.
M: Me, too. This is unusual for March. I don’t remember it ever being so hot and dry in March before.
W: My mother and I have just moved here from Indiana. It is pretty cold there.
M: Yes, I’ve heard of that.
W: Do you think we’ve missed the bus
M: Take it easy. It never comes exactly on the half hour like it should.

How often should the bus come to their stop().

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