Size analysis was carried out on a sample of gravel. The data for mass fraction and average particle diameter of the fraction is given in the following table: xD (mm) 0.2 5 0.4 10 0.6 15 0.8 20 The volume surface mean diameter, D, of the sample ------ (mm) a) 4.25 mm b) 5.53 mm c)6.25 mm d)2.56 mm A