Find the acceleration of the block.

C. 7 m/s²

Explanation:

The horizontal forces acting on the block are the 80 N tension, the 60 N tension, and friction.  Apply Newton's second law:

∑F = ma

80 N − 60 N − F = (10 kg) (1 m/s²)

20 N − F = 10 N

F = 10 N

If S₁ goes away, the acceleration becomes:

∑F = ma

80 N − 10 N = (10 kg) a

70 N = (10 kg) a

a = 7 m/s²

neither accurate nor precise.