题目内容：
根据以下材料，回答题Passage Three
If you have ever been rock climbing ,then you will know that it is not a very easy sport. In fact you proba-bly felt quite defeated when you first tried to do any serious climbing. Indoor and outdoor rock climbing are both extremely demanding and, like anything else, require practice, to get good at. What most people don't
know is that the sport of rock climbing is one of the most physically and mentally demanding sports there is.
There are three basic types of rock climbing. Top roping is probably one of the most common types and is basically climbing with a partner. The second type, which is very similar to top roping, is called lead climb-ing. In both of these types of rock climbing the climber can sit off the wall and rest on the rope.
The third type of rock climbing is called bouldering. Bouldering is a type of free climbing without any ropes. This is the most demanding of all climbing types. The climber must be able to complete the climbing without taking a rest on the rope.
Believe it or not, climbing is said to be about 75%legs and only 25% arms. To climb efficiently and suc-cessfully, a person needs to have a wonderful technique. One of the major rules of rock climbing is to always have three points touching the wall, whether it is both feet and one hand, or one foot and both hands, as it is
much easier to have your weight rest on three points than on two. Another important idea to grasp is that your arms in most cases will do the climbing；they only hold you into the wall so that your legs are actually push-ing you upward. Also, the closer you are to the wall, the easier it is to climb.
Rock climbing may sound a bit too extreme for the everyday person but it really is an amazing workout.Once you get into the sport, and learn how to position your body and rest your weight, then you can begin to deal with some difficult problems. The great thing about rock climbing is that it is mentally challenging as well. You are constantly analyzing the way your body moves and how to do certain moves on the wall. For any-one who wants to get into shape, rock climbing is a fun and effective way to exercise muscles.
( ) What does the author think of rock climbing? A．It is an easy sport.
B．It is quite dangerous.
C．It is challenging.
D．It is extremely difficult.
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