4. (a) Tim and Tony are out taking walks. Let x(t) be the distance between them at time t. If Tim is currently 1 mile south of Tony, in which of the following situations is dx dt negative right now? List all. A. Tim is walking south at a rate of 3 mph, while Tony is walking south at a rate of 2.5 mph. B. Tim is walking south at a rate of 2.5 mph, while Tony is walking south at a rate of 3 mph. C. Tim is walking north at a rate of 3 mph, while Tony is walking north at a rate of 3.5 mph. D. Tim is walking north at a rate of 3 mph, while Tony is walking south at a rate of 3.5 mph. E. Tim is walking south at a rate of 3 mph, while Tony is walking north at a rate of 3.5 mph. (b) Ana, Jelena, and Novak are working on the following problem. Gravel is being dumped from a conveyor belt, and its coarseness is such that it forms a pile in the shape of a cone whose base diameter and height are always equal. When the pile is 10 ft high, the gravel is being dumped at a rate of 30 ft3/min. At this moment, how quickly is the height of the pile increasing? They've decided to use the variables V = volume of gravel, h = height of pile, and t = time. Now, they're trying to decide on a strategy. Ana says, "We're looking for dh dt , so we should write h in terms of t and then differentiate." Jelena says, "We're looking for dh dt , but we know dV dt , so we should first relate h and V and then differentiate implicitly with respect to t." Novak says, "We're looking for dh dV , so we should first relate h and V and then differentiate implicitly with respect to V ." Which of the three has a workable strategy?