@srestha ma'am, Here it is mentioned in the question that only area 51 has extra terrestrials.. So if extra terrestrials is present in any area, then it can be present only in area 51, so for all has to be used. Because if we use there exists, then the predicate will become true even if extra terrestrials is present in an area that isn't area 51 as well as if extra terrestrials is present in the area 51.