The Quantity Which Does Not Change in Magnitude from That $\overrightarrow{\mathbf{a}}$

Multiple Choice

The quantity which does not change in magnitude from that observed in system S when observed in system S' moving away from system S at speed v is:

A) m $\overrightarrow{\mathbf{a}}$ .
B) m $\overrightarrow{\mathbf{v}}$ .
C) ( $\gamma$ - 1) mc2.
D) E2 -p2c2.
E) x2 + y2 + z2.