It takes 4 weeks for a team of 5 professional Window Washers working regular full time hours to completely clean every window of the Empire State building. The building’s owner demands that all the windows always be clean.Yet even if the 5 washers work consistently throughout their regular work week,they will not be able to finish cleaning all the windows before some windows will need cleaning .

Which of the following statements as must be true on the basis of the statements above ?

A) If an empire state building window is to be kept clean, it must be cleaned by a professional cleaner.
B)The owner’s demand for proper cleaning of all the windows will never be fulfilled.
C) If a team of 5 window washers cleans all the empire state building’s windows in less than 4 weeks, some windows will not be properly cleaned.
D) In order to ensure that all of the Empire State Building’s windows are clean, the owner must have his window washers work overtime.
E)Some Empire State Building windows must be cleaned more frequently than once every four weeks if they are to be kept clean.

My Ques: How do we know if overtime is needed?