The correct answer is D) areas where direct impingement from deck mounted machines cannot be satisfactorily reached. Bottom mounted crude oil washing machines are primarily used for cleaning hard to reach areas of the tank, where the direct impingement from deck mounted machines is not sufficient. They are designed to provide coverage and agitation in areas that cannot be properly cleaned by the deck mounted units alone. The other options are incorrect because A) refers to vertical surfaces which are typically cleaned by the deck mounted machines, B) is too broad as the bottom mounted machines are used for only a portion of the tank cleaning, and C) is incorrect as the bottom mounted machines do not necessarily clean the entire tank bottom.