Which of the following are context free?
$$A = \left\{a^nb^n a^mb^m \mid m, n \geq 0\right\}$$
$$B = \left\{a^mb^n a^mb^n \mid m, n \geq 0\right\}$$
$$C = \left\{a^mb^n\mid m \neq 2n, m, n \geq 0\right\}$$

A

A and B only

B

A and C only

C

B and C only

D

C only