A group of students is investigating whether the smoothness of a surface affects the force of friction. Each student pushes a book on two different surfaces. One surface is ice and the other is concrete. The distance traveled by the book on each surface is shown. Experimental Observations
Student 1 Student 2 Student 3 Student 4
Distance traveled by book on ice 23 cm 21 cm 24 cm 22 cm
Distance traveled by book on concrete 15 cm 18 cm 16 cm 17 cm

Based on the table, which conclusion is correct about the average distance traveled?
A. It is less on concrete.
B. It is greater on concrete.
C. It is less when the book is pushed harder.
D. It is independent of the type of surface.