A

\[\mathbb{R}-\left\{2k\pi; k \in \mathbb{Z}\right\}\]

B

\[\mathbb{R}-\left\{k\pi; k \in \mathbb{Z}\right\}\]

C

\[\mathbb{R}-\left\{\frac{k\pi}{4}; k \in \mathbb{Z}\right\}\]

D

\[\mathbb{R}-\left\{\frac{k\pi}{3}; k \in \mathbb{Z}\right\}\]

E

\[\mathbb{R}-\left\{\frac{k\pi}{2}; k \in \mathbb{Z}\right\}\]