If E is the set of months having 28 days in a common year, what kind of set is E?
Correct answer: A
The governing concept is classification by cardinality, meaning by the number of distinct elements in a set. In a common year, February has exactly 28 days, whereas every other month has either 30 or 31 days. Therefore E contains exactly one member: February. Any set with precisely one element is called a singleton set, so option A is correct. It is not empty because February satisfies the condition. It is not infinite because the calendar has a finite number of months. It is not undefined because the phrase “common year” clearly specifies the relevant calendar case. The leap-year situation does not alter the answer because the question explicitly excludes leap years.