2. A 2.0 kg box (A) collides elastically with a stationary 4.0 kg box (B). Which of the following is true? (a) A moves backward at half its initial speed, and B moves forward at half of A's initial speed. (b) A moves backward at one-third of its initial speed, and B moves forward at two-thirds of A's initial speed. (c) A and B move forward at half A's initial speed. (d) A and B move forward at one-fourth of A's initial speed