Final answer:
The forces on the cat and the turtle are unbalanced because the force of the cat's weight is greater than the opposing force. The rock cannot be moved because the forces between you and the rock are balanced.
Step-by-step explanation:
The best explanation for the situation where two forces are balanced on the cat but unbalanced on the turtle is that the force of the cat's weight is greater than the opposing force. This is because the cat is larger and therefore has a greater weight compared to the turtle. In other words, the force of gravity pulling the cat down is stronger than the opposing force acting on the cat.
As for the situation where you cannot make a round rock roll even when pushing hard, the best explanation is that the forces between you and the rock are balanced. This means that the force you are applying is being countered by an equal and opposite force. Therefore, there is no resulting motion or movement of the rock.