It is given that $X_{1}, X_{2}, \cdots X_{M}$ are $M$ non-zero, orthogonal vectors. The dimension of the vector space spanned by the $2 M$ vectors $X_{1}, X_{2}, \cdots X_{M},-X_{1},-X_{2}, \cdots-X_{M}$ is

- $2 M$
- $M+1$
- $M$
- dependent on the choice of $X_{1}, X_{2}, \cdots X_{M}$