The Iron Oxides and Manganese Oxides That Produce Desert Varnish

The iron oxides and manganese oxides that produce desert varnish on the exterior surfaces of rocks are derived from ions ____________.

A) in the interiors of the rocks, which are transported outward by the capillary action of water
B) in the interiors of the rocks, which are transported outward by microorganisms
C) that occur naturally in rain water and precipitate out as oxides after the rain evaporates
D) that occur naturally on the surfaces of carbonate and silicate rocks