70. The minimum concentration of oxygen necessary to sustain fish life in an aquarium is 4 mg/L. Assuming the density of theaquarium water solution is 1.00 g/mL, what is the minimum concentration of O2 expressed in parts per million (ppm).
(A) 2 ppm
(B) 4 ppm
(C) 2000 ppm
(D) 250 ppm
(E) 0.004 ppm