Determine whether the following statements are true and give an explanation or counterexample. 

a. The distance traveled by an object moving along a line is the same as the displacement of the object.

b. When the velocity is positive on an interval, the displacement and the distance traveled on that interval are equal.

c. Consider a tank that is filled and drained at a flow rate of V'(t) = 1 - t²/100 (gal/min), for t ≥ 0, where t is measured in minutes. It follows that the volume of water in the tank increases for 10 min and then decreases until the tank is empty.

d. A particular marginal cost function has the property that it is positive and decreasing. The cost of increasing production from A units to 2A units is greater than the cost of increasing production from 2A units to 3A units.