If $\alpha$, $\beta$ and $\gamma$ are the angles which a line makes with positive directions of x, y and z axes respectively, then which of the following is not true ? (A) $\cos^2 \alpha + \cos^2 \beta + \cos^2 \gamma = 1$ (B) $\sin^2 \alpha + \sin^2 \beta + \sin^2 \gamma = 2$ (C) $\cos 2\alpha + \cos 2\beta + \cos 2\gamma = -1$ (D) $\cos \alpha + \cos \beta + \cos \gamma = 1$