First of all, we know that a professional volleyball player can jump an average of 2.5 times higher than the average person which means that the player could jumps 2.5 times higher than the normal do. For example, if the normal people jumps 3 inches height, the professional player could jumps 2.5(3)=7.5 inches height. In this case, if an an average person can jump 2x inches high which leads to that the professional player could jumps 2.5(2x) higher. As a result, C is your final answer. Hope it help!