18) When the kinetic energy of a body changes with time, what does this change represent? (A) The work done by some force. (B) The work done by a net force. (C) The work done by some average force. (D) The work done by an irregular force. 19) A mass m has velocity v at a position X, what will be the work done if the distance traveled is quadrupled? (A) mv^2 (B) mv? (C) 3/2 mv^2 (D) 2mv