Which level of organization is made up of similar cells working together

Mason joined the water polo team at his school. as part of his conditioning, mason's coach requires a 25-minute run during practice. mason's heart rate reaches 125 beats per minute, but his target heart rate zone is 138—183 beats per minute. what should mason do to get the most benefit for his heart? increase the amount of time he runs increase the speed at which he runs decrease the amount of time he runs decrease the speed at which he runs