[单选题]
04.23: If two molecu2tmr80g8c5s vbi 9ew(n 6q vvulwxi)xbv( dqhe r9 ,6j+es in a gas collide, then
A. total momentum is always unchanged.
B. velocities are always exchanged
C. both molecules always speed up.
D. total kinetic energy always increases.
E. velocities are always unchanged.