4. A 2.5 g sample of groundwater was found to contain 5.0 ng of Cr6+, what is the concentration of Cr6+in parts per million? Cr: 52.0 u
(A)2.0 ppm
(B)0.2 ppm
(C)0.02 ppm
(D)0.002 ppm
(E)0.0002 ppm