Which of the Following Processes Is Consistent with the Law $\mathrm{C}$

Multiple Choice

Which of the following processes is consistent with the law of conservation of four-momentum? (Answer $\mathrm{C}$ if it is consistent with the law; $F$ if it violates it.)
-(a) A particle moving eastward collides with and sticks to an identical particle at rest. The resulting single particle remains at rest but is more massive than the sum of the original particles' masses.

A) C
B) $F$