How does reflection differ from refraction and diffraction?

For which pair of objects would adding the same a

Under the action of a constant force an object accelerates at 7.8 m/s2. what will the acceleration be if (a) the force is halved? (b) the object's mass is halved? (c) the force and the object's mass are both halved? (d) the force is halved and the object's mass is doubled?

The combining of light nuclei is called blank. blank as in not actually blank. you know what im tryin to say.