Which of the following situations would cause the greatest decrease in the motion of molecules in a system?

A. Q = - 50 J, W = - 50 J B. Q = + 50 J, W = + 50 J C. Q = - 50 J, W = + 50 J D. Q = + 50 J, W = - 50 J

The answer is A. Q = - 50 J, W = - 50 J. A decrease in the motion of molecules in a system can be interpreted as a decrease or release in energy which means that energy should be negative. For the formula of energy:

U = Q + W

U is negative for the case of A. This gives the greatest decrease in the molecules' motion.