【小题1】--_________________（多长）is the river?
--It's about 3,000 kilometres.
【小题2】They could travel at_______________________(两倍多) the speed of sound.
【小题3】_________________(确保)that you get there in time.
【小题4】They walked for a while and_______________________（停下来休息）a rest.
【小题5】The box is________heavy for me________（太……而不能……）carry.