If $\sum _{\mathrm{i}=1}^{2\mathrm{n}} {\cos }^{-1}{\mathrm{x}}_{\mathrm{i}}=0$, then $\sum _{\mathrm{i}=1}^{2\mathrm{n}} {\mathrm{x}}_{\mathrm{i}}$ is

• A

  n

• B

  2n

• C

  $\frac{\mathrm{n}(\mathrm{n}+1)}{2}$

• D

  none of these