Based on the concentrations of aqueous Fe, and assuming each horizon had equal amounts of Fe(III) to begin with, does the A, Bt1 or Bt2 horizon enter the Fe(III) reduction stage first? a. horizon
b. Bt1 horizon
c. Bt2 horizon

A Horizon.

Explanation:

A Horizon enters the Fe(III) reduction stage first because more concentration of microbes present in A horizon which started Fe(III) reduction stage first than the B horizon. So if each horizon had equal amounts of Fe(III) then Fe(III) reduction stage first started in A horizon and provide iron to the plants. A horizon is also refers to the top soil layer which contains dark colored decomposed organic matter, which is also called "humus".

answer ( groundwater);