One of the most difficult but rewarding pastimes is mountain climbing. The modern climber must (1) ... many different skills. Rock climbing (2) ... a combination of gymnastic ability, imagination and observation, but perhaps the most necessary skill is being able to (3) ... out how much weight a particular rock will support. Mountaineers climb in groups of three or four, each climber at a distance of approximately six meters from the next. Usually one person climbs while the other climbers (4) ... hold of the rope. There is no (5) ... that the most experienced climber goes first. He shows the other climbers which (6) ... to go, making the rope secure so that it is (7) ... for the others to follow. With much mountain climbing, snow skills (8) ... a very important part. The number of dangers (9) ... by climbers is almost endless. And the (10) ... of oxygen at high altitudes makes life even more difficult fot mountaineers.