If $1, \alpha, \alpha^{2}, \alpha^{3}, \ldots, \alpha^{n-1}$ are the nth roots of unity, then the value of $(2-\alpha)\left(2-\alpha^{2}\right)\left(2-\alpha^{3}\right) \ldots\left(2-\alpha^{n-1}\right)$ is
(a) $2^{n}$
(b) $1-2^{n}$
(c) $2^{n}-1$
(d) 2