Why did Sumer, Harappa, and the Chinese civilization all form in river valleys

You think all professors are uncoordinated, but then you see your social psychology professor make a diving catch down the left-field line at an intramural softball game against the arts department team. you also notice that your professor hits lead-off for the team and is able to score from second base on a ground-out. you maintain your original stereotype of professors as uncoordinated by deciding that this one individual is an exception to the rule because he is a “young professor.” this is an example of