Greg is trying to push a box of books across the floor of his room. The box of books doesn't move because the forces on the box are ____________. When his little brother walks in and starts pushing with him, the box begins to slide across the floor. His brother's pushing, combined with his, makes the forces on the box ______________. A. unbalanced; balanced B. balanced; unbalanced C. shifting; still D. not acting on the box; moving