Question 18 4 pts A stack of books rests on a level frictionless surface. A force Facts on the stack, and it accelerates at 4.0 m/s2. A 1.0 kg book is then added to the stack. The same force is applied, and now the stack accelerates at 2.0 m/s2. What was the mass of the original stack? O 3.0 kg O 2.0 kg O none of the above 1.0 kg