If d 1 is the depth of cutting, d 2 is the height of the bank from bed level r 2 : 1 and r 1 : 1 are the slopes in filling and cutting respectively, the horizontal distance n between the bed and bank, is

• A. x = r 1 d 1
• B. x = r 2 d 2
• C. x = d 1 / r 1
• D. x = d 2 / r 2