Why are some of the geological time periods not represented in the strata forming the grand canyon?

Some geological time periods are not represented in the strata forming the Grand Canyon, because the evolutionists believed that there are large "time-breaks". However, others believe that there is "erosion [that] was very rapid, facilitated in many cases by erosion occurring in soft, ‘non-hardened’ rock". This generally leads to evidence of "The Flood", and the like

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The correct answer is  b)