Given above is a simplified model of phosphorus cycling in a terrestrial ecosystem with four blanks (A-D). Identify blanks.

Phosphorus is a major constituent of the biological membrane, nucleic acids and cellular energy transfer system. Detritus is leaf litter and other organic matter intermixed with soil, which is referred to as humus. Rocks are composed of one or more minerals. Primary producers are organisms in an ecosystem that produce biomass from inorganic compounds (autotrophs). Litterfall is dead plant material, such as leaves, bark, needles, and twigs, that has fallen to the ground. This detritus or dead organic material and its constituent nutrients are added to the top layer of soil, commonly known as the litter layer .