$$2$$-chloro-$$2$$-methylpentane on reaction with sodium methoxide in methanol yields:

The product of the reaction given below is :

The species in which the $\mathrm{N}$ atom is in a state of $s p$ hybridization is :

If $$0 \le x < 2\pi $$, then the number of real values of $$x$$, which satisfy the equation $$\,\cos x + \cos 2x + \cos 3x + \cos 4x = 0$$ is: