First, consider the situation and the effect of "resultant force" on a body. Which of Newton's Laws applies to this situation? If the resultant force combines inertia, gravity, magnetism, push/pull, etc. and is opposite the current motion, then the current motion will become slower and slower (and might even cause the motion to go in the opposite direction). Imagine the cartoon of a guy trying to slow a boat that is about to go over the waterfalls; he applies a force in a direction opposite to the current motion.
Now, it is the resultant force that determines the answer. Suppose a car is going down a hill -- gravity is pulling it down, but the car's breaks SLOW the speed because the resultant force (all considered) is in the opposite direction of motion.