If scientists believe Earth is actually 4.6 billion years old, why haven't scientists found older oceanic crust?

So far scientists have not found a way to find the exact age of the Earth directly from Earth rocks because Earth's oldest rocks have been recycled and destroyed by the process of plate tectonics. Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents

Explanation:

the calvin cycle has three stages. in stage 1, the enzyme rubisco incorporates carbon dioxide into an organic molecule. in stage 2, the organic molecule is reduced. in stage 3, rubp, the molecule that starts the cycle, is regenerated so that the cycle can continue

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."