To compensate the loss of tractive force of vehicles along curves of radius R, the percentage reduction of gradient, is

• A. <img src="/_files/images/civil-engineering/highway-engineering/31-16-90-1.png">
• B. <img src="/_files/images/civil-engineering/highway-engineering/31-16-90-2.png">
• C. <img src="/_files/images/civil-engineering/highway-engineering/31-16-90-3.png">
• D. <img src="/_files/images/civil-engineering/highway-engineering/31-16-90-4.png">
• E. <img src="/_files/images/civil-engineering/highway-engineering/31-16-90-5.png">