The answer to your question would be B. 15-20%
<span>Ab Roller Evolution is made of metal. It supports neck, head and upper body during workout. It is an upper body trainer: works chest and core. You can do push ups, sit ups, dips. By providing improved support, it reduces stress and strain on body.</span>
<span />Description above is from Amazon.
It is a good idea but has some problems with its design. There is no way to secure it on the floor during exercise. So it moves around when pushed against and would not provide support. A normal crunch will likely work out better.
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