If scientists wanted to date the oldest rocks on Earth's surface, why wouldn't they use rocks formed at the Mid-Ocean ridges ?

Explanation:

This is because rocks formed at the mid ocean ridges are young rocks i.e the mid ridges is use to date young rocks. Rocks farther from the mid ridges are older than rocks near the mid ridges and this can also be as a result of the oceanic crust near the mid ridge are younger than the oceanic crust far away from the mid ridge.

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