Each of 16 students measured the circumference of a tennis ball by four different methods, which were: a: estimate the circumference by eye b: measure the diameter with a ruler, then compute the circumference c: measure the circumference with ruler and string d: measure the circumference by rolling the ball along a ruler the results (in cm) are given below, in increasing order for each method: a: 18.0, 18.0, 18.0, 19.0, 22.0, 22.0, 22.5, 22.9, 24.0, 24.0, 25.0, 25.0, 25.0, 25.0, 26.0, 26.4 b: 18.8, 18.9, 18.9, 19.8, 20.1, 20.4, 20.4, 20.4, 20.6, 20.5, 21.2, 21.9, 22.0, 22.0, 22.0, 23.6 c: 20.2, 20.5, 20.5, 20.7, 20.8, 20.9, 21.0, 21.0, 21.0, 21.0, 21.0, 21.5, 21.5, 21.5, 21.5, 21.6 d: 20.0, 20.0, 20.0, 20.2, 20.2, 20.5, 20.5, 20.7, 20.7, 20.7, 21.0, 21.5, 21.5, 21.6, 22.1, 22.3