Before the rope between AB burns out, according to the equilibrium conditions, the elastic force of the lower spring is F 1=mCg=3mg, the elastic force of the upper spring is F2=(mA+mB+mC)g=6mg, and the tensile force of the rope between AB is T = (MB+MC) g = 5 mg.

Before the rope between AB burns out, the elasticity of the two springs has not changed, so

Right A: At this time, the resultant force acting on A is equal to the original tensile force T of the rope. If the direction is opposite, that is, the direction is upward, then -5mg=maA and AA =-5 g. 。

Right B: At this time, the resultant force acting on B is equal to the pulling force T of the original rope, and the direction is opposite, that is, the direction is downward, so 5mg=2maB, aB=2.5g 。

Right C: Since the elastic force of the spring has not changed, the stress of C has not changed, so AC = 0.

So choose B.