4. Water is flowing at 4.0 m/s in a circular pipe. If the diameter of the pipe decreases to 1/3 its former value, what is the velocity of the water downstream? (A) 1.0 m/s, (B) 2.0 m/s, (C) 8.0 m/s, (D) 16 m/s, (E) 36.0 m/s.