The correct answer is muscles can only pull.
The movement of arm takes place by the relaxation and contraction of muscles. Muscles in animals are in pair because muscles can only pull, when a pair of muscle is present in animals one muscle relaxes and the other one contracts. An example of this is biceps and triceps of arm. The triceps of the arm relaxes while the biceps of the arm contracts, this movement helps to lift the arm. The reverse also takes place in the arm.