Why is the standard model unable to predict a particle's mass?
A. It is not precise enough to predict mass.
B. It lacks a mathematical model to predict mass.
C. It can observe only first-generation leptons.
D. It can observe only first-generation quarks.

Arailroad car collides with and sticks to an identical railroad car that is initially at rest. after the collision, the total kinetic energy of the two cars is a) the same as before. b) half as much as before. c) one third as much as before. d) one fourth as much as before. e) twice as much as before.