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

• A
  $A$-Rock minerals, $B$-Detritus, $C$-Litter fall, $D$-Producers
• B
  $A$-Litter fall, $B$-Producers, $C$-Rock minerals, $D$-Detritus
• C
  $A$-Detritus, $B$-Rock minerals, $C$-Producers, $D$-Litter fall
• D
  $A$-Producers, $B$-Litter fall, $C$-Rock minerals, $D$-Detritus