When oxygen is depleted, the citric acid cycle stops. What could we add to the system to restore citric acid cycle activity (other than oxygen) ?

NAD+, FAD.

Explanation:

The citric acid cycle is popularly known as the Kreb's cycle. The cycle involve the oxidation of acetyl-CoA to produce energy. The Kreb's cycle is a chemical process that produces produces two carbon dioxide molecules, NADH, FADH2 and one ATP.

When oxygen is depleted, the citric acid cycle stops, apart from oxygen NAD + and FAD could be added to the system to restore citric acid cycle activity. NAD + acts as an electron acceptor.

Citric acid cycle/Kreb's cycle is an aerobic process that occurs in the mitochondria and produces thirty-six (36) ATPs.