Which of the following examples does not illustrate Newton's first law of motion?
A. a glass of water sits motionless on a counter
B. a rocket engine accelerates a spacecraft along its trajectory
C. two students push on opposite sides of a desk, but it does not move
D. a hockey puck slides across smooth ice in a straight line at constant speed
PLEASE HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 50 pts!!!!!!!!!