Two Blocks Are at Rest on a Horizontal Frictionless Surface

Multiple Choice

Two blocks are at rest on a horizontal frictionless surface with a compressed spring between them.The 5.0-kg block accelerates when the string holding the system in compression is cut and the 10-kg box accelerates in the opposite direction.What is the magnitude of the force from the expanding spring on the 10-kg box when the 5.0-kg box is undergoing an acceleration of 9.0 m/s²?

A) 22.5 N
B) 90 N
C) 135 N
D) 45 N
E) ​0 N