A rock is thrown vertically upward with initial speed vo. Assume a friction force proportional to -v, where u is the velocity of the rock, and neglect the buoyant force exerted by air. Which of the following is correct? A. The acceleration of the rock is always equal to g. B. The acceleration of the rock is equal to g only at the top of the flight. C. The acceleration of the rock is always less than g. D. The speed of the rock upon return to its starting point is vo. E. The rock can attain a terminal speed greater than u, before it returns to its starting point.