As algae spread in freshwater ponds and lakes, water quality decreases; thus, reducing the nutrients such as phosphorus, that enter a body of water, is central to reducing algae, which thrive on certain pollutants.

A. nutrients such as phosphorus, that enter a body of water, is central to
B. nutrients that enter a body of water such as phosphorus is key to
C. nutrients, such as phosphorus, that enter a body of water plays a major role in
D. nutrients such as phosphorus that enter a body of water are a major part of
E. nutrients such as phosphorus which can enter a body of water, hold the key

C OE says "Reducing (the nutrients)" is the singular subject if so then why there is "that enter" why not "that enters" in OA