Walking is not the most exciting form of exercise a person can take on for fitness. Yet it is low impact and requires no fancy equipment. That makes walking a simple activity for those new to exercise. Walking is less likely than other exercises to cause injury to leg tendons and muscles. All it requires is a good pair of sneakers—no cables, stylish outfits, or weights required. Of course, a person could increase the benefit of a workout by adding weights or speed walking. How does the third sentence ("That makes walking...") support the main idea of the paragraph?

It points out that walking is a good introduction to exercise for beginners.
It says that walking is too easy for people who have experience with exercise.
It points out that beginning exercisers may not like walking because it is too easy.
It says that walking is less costly than other activities because it needs no equipment.