A 4-Kg Mass and a 5-Kg Mass Are in Contact

Multiple Choice

A 4-kg mass and a 5-kg mass are in contact with each other while resting on a smooth table. A 20-N external force is applied to the 4-kg mass (in a direction toward the 5-kg mass) . The contact force between the two masses is

A) 20 N.
B) 9 N.
C) 0 N.
D) 11 N.