Problem Three

The ks, rs, q1, q2 all cancel out.
You are left with F2/F1 = 9/1
The answer is C
Problem Four
This problem is a bit harder. In the first place one of the charges must be - otherwise the force will not be an attractive one which is what the question implies. So let q < 0.
That aside, I don't know how the acceleration could be a constant. As the distance decreases the force increases and therefore so does the acceleration. But it is is not a constant.
I would say that the answer is B.