By midday, the small party of boys, led by their schoolmaster, had reached a height of 2,5

By midday, the small party of boys, led by their schoolmaster, had reached a height of 2,500 feet. At this point the party had to stop climbing because one of the boys became seriously ill. The only thing the master could do was to return to the mountain hut where they ha& spent the night. From there, he telephoned the police. As no rescue party could reach the boy quickly enough, the Royal Air Force Rescue Center sent a helicopter (直升飞机) with a policeman and a doctor on board. The helicopter soon arrived on the scene, but the sides of the mountain were so steep that it could not land. A helicopter usually lands on four wheels, but it can land on two. However, the slope was too steep even for that. The pilot, there fore, kept the helicopter in the air with only one wheel touching the mountain side while the party carried the boy on board.

The group of boys had to stop because ______.

A．they had reached the peak

B．one of the boys was badly injured

C．one of the boys was sick

D．they had to return to the hut