newtonian mechanics - Two balls A and B of the same size are dropped from the same point.

If the mass of A is greater of the two, and if air resistance is the same on both, which ball will reach the ground first/simultaneously?

I thought that since the acceleration acting on them is same, both will reach the ground simultaneously. But the answer in the book says that ball A (the one having greater mass) will reach first. I mean, isn't this what people thought before (I guess it's true) Galileo performed his experiment, until they were proven wrong?!