单项选择题A．Big waves pose a threat to commercial navigation.B．The river current never flows faster ten miles per hour.C．The river reverses its flow several times a day.D．High tides can create sudden and unexpected rapids.

单项选择题A．To tell him whether her car is repaired.B．To find out whether he wants to go canoeing C) To find out what kind of food he is bringing.C．To tell him what time they are leaving.