Correct!

The best answer is H.

According to the figure, concentration of the key quantity is 1.0 ppm both at 0 and 20 cm.

F. Ferrous iron (Fe²⁺)

Incorrect. Ferrous iron's concentration is 0.5 ppm at 0 cm and 4.0 ppm at 20 cm.

G. Oxygen (O₂)

Incorrect. Oxygen's concentration is 2.0 ppm at 0 cm and 0.0 ppm at 20 cm.

H. Carbon dioxide (CO₂)

Correct. Carbon dioxide's concentration increases from 1 ppm at 0 cm to

3 ppm at 15 cm, then decreases to 1 ppm at 20 cm.

J. Sulfate (SO₄^2–)

Incorrect. Sulfate's concentration is 7.0 ppm at 0 cm and 3.0 ppm at 20 cm.