Phương trình 2 cosx - căn 2 =0 có tất cả các nghiệm là

A. $\left[\begin{array}{l}x=\frac{3\pi }{4}+k2\pi \\ x=-\frac{3\pi }{4}+k2\pi \end{array}\right.\left(k\in \mathbb{Z}\right)$  

B. $\left[\begin{array}{l}x=\frac{\pi }{4}+k2\pi \\ x=-\frac{\pi }{4}+k2\pi \end{array}\right.\left(k\in \mathbb{Z}\right)$

C. $\left[\begin{array}{l}x=\frac{\pi }{4}+k2\pi \\ x=\frac{3\pi }{4}+k2\pi \end{array}\right.\left(k\in \mathbb{Z}\right)$  

D. $\left[\begin{array}{l}x=\frac{7\pi }{4}+k2\pi \\ x=-\frac{7\pi }{4}+k2\pi \end{array}\right.\left(k\in \mathbb{Z}\right)$