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 camp (营地) where they had spent the night. From there, he telephoned the police. As no rescue party (救援队) 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 (陡峭的) even for that. The pilot, therefore, kept the helicopter in the air with only one wheel touching the mountainside while the party carried the boy on board.2.2.2 A boy was ill ( ).

A. when the team started climbing

B. when the team reached the top of the mountain

C. when they reached a height of 2500 feet

D. when they returned to their mountain camp