Following curve represents 3 entities in a following river in which sludge was added.Curve X & Z represent (respectively)

Question

$\begin{array}{1 1}(a)\;\text{ Sludge & Nitrogen } \\ (b)\;\text{Oxygen & Nitrogen} \\(c)\;\text{Oxygen & Microbes} \\(d)\;\text{Nitrogens & Microbes}\end{array}$