Elastic Collisions in One Dimension: a Block of Mass M

Elastic collisions in one dimension: A block of mass m = 8.40 kg, moving on a horizontal frictionless surface with a speed 4.20 m/s, makes a perfectly elastic collision with a block of mass M at rest. After the collision, the 8.40 kg block recoils with a speed of 0.400 m/s. In the figure, the blocks are in contact for 0.200 s. The magnitude of the average force on the 8.40-kg block, while the two blocks are in contact, is closest to

A) 193 N.
B) 185 N.
C) 176 N.
D) 168 N.
E) 160 N.