11) What is the negation of "all perfect numbers are even"? (It is not necessary to know what a perfect number is.)

(A) All perfect numbers are odd

(B) there is at least one odd perfect number

(C) there is at least one even number that is not perfect

(D) no odd number is perfect

(E) no even number is perfect.
11) The answer is (B).

Indeed, the negation of the given statement is that not all perfect numbers are even, and therefore there exists at least one perfect number that is odd.