Hi :) I don’t really understand why it’s A

A

Explanation:

1. When the block moves across a table top, F(pulling) = - F(frictional).

Sum of this forces = 0, so the block moves with uniform speed.

2. When the block is pulled on top of the table covered with beads

F(pulling) > - F(frictional).

So, the sum of forces (∑F) is a number that is more than 0 and directed to the direction of movement.

So,  a = ∑F / m is positive and constant. Speed is increasing because

v(t) = v(0)+at

a is constant and  directed forward.

That means a is acceleration, and constant.