The diagram shows object N placed one on top of object M. M has a mass of 4 kg and N has a mass of 2 kg. Assume that all the contact surfaces are frictionless, what is the acceleration of N when M is
pulled by a force of 20 N?
A. 0 m/s²
B. 4 m/s²
C. 3.3 m/s²
D. 10 m/s²