A Particle Moves Along The $x$ Axis the Slope of the Arrow Representing an Object's Four-Momentum

Multiple Choice

A particle moves along the $x$ axis. The slope of the arrow representing an object's four-momentum on an energy-momentum diagram is equal to

A) The particle's speed.
B) The particle's $x$ -velocity.
C) The inverse of the particle's speed.
D) The inverse of the particle's $x$ -velocity.
E) The particle's energy.