A 5-Kg Shark Swimming at a Speed of 1 M/s

A 5-kg shark swimming at a speed of 1 m/s swallows an absent-minded 1-kg fish swimming toward it at 4 m/s. The speed of the shark after this meal is

A) 2/3 m/s.
B) 1/5 m/s.
C) 1/2 m/s.
D) 3/2 m/s.
E) 1/6 m/s.