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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

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