ANS:C - floating gradient

Gradient :

It is expressed as the ratio of the difference of heights between the two points and the distance between them.

Eg: Now when you are going up on the hill by road, then you need more tractive effort. But when you are coming down, the tractive effort is negative, means your car will speed downwards. Even while going Straight a car need some amount of tractive effort. But there is a definite gradient in which you will need no tractive effort. Remember, it is not negative tractive effort but just the amount that is required to maintain the speed. So that specific gradient in which you can maintain a specific speed will be called floating gradient.