15．听下面一段材料，回答19～21题．
19．What do we know about Mary？
A．She can't decide where to go．
B．She has too many pictures to show．
C．She can't think of a topic forher composition．
20．What does Jack suggest Mary do？
A．Take a trip． B．Ride a horse C．Write about her trip
21．Why does Jack have to leave？
A．He's got to write his owncomposition．
B．He's goingto pack for the trip．
C．He's going to pick up somephotographs．
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