The third sentence ("That makes walking …") support the central idea of the paragraph as B. It points out that walking is a good introduction to exercise for beginners.
It should be noted that the central idea simply means the main idea that's illustrated in the story. It should be noted that this is important to the development of the story.
The least thrilling exercise a person can do to get fit is walking. However, it has a modest impact and doesn't call for expensive gear. Because of this, walking is an easy exercise for beginners. Walking has a lower risk of harming the tendons and muscles in the legs than other workouts. All you need are a good pair of sneakers; there's no need for wires, fashionable attire, or weights. Of course, adding weights or speed walking to an exercise could improve its benefits.
In this instance, the third sentence ("That makes walking...") supports the main idea of the paragraph by emphasizing that it is a good way for beginners to begin exercising.
In conclusion, the correct option is B.
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