Which of the following are not regular?
(A) Strings of even number of a’s.
(B) Strings of a’s, whose length is a prime number.
(C) Set of all palindromes made up of a’s and b’s.
(D) Strings of a’s whose length is a perfect square.
(1) (A) and (B) only
(2) (A), (B) and (C) only
(3) (B), (C) and (D) only
(4) (B) and (D) only
(A) Strings of even number of a’s.
(B) Strings of a’s, whose length is a prime number.
(C) Set of all palindromes made up of a’s and b’s.
(D) Strings of a’s whose length is a perfect square.
(1) (A) and (B) only
(2) (A), (B) and (C) only
(3) (B), (C) and (D) only
(4) (B) and (D) only