Under natural conditions, oceans are
self-cleaning systems
ideal waste-dumping grounds
of no use to humans
most damaged by air pollution

Under natural conditions, oceans are self-cleaning systems.

Explanation:

The oceans usually do not house any waste, unwanted material in themselves. The ocean waves either dispose off the waste material back on the shores, or the saline ocean water decays the degradable material. The marine organisms are also majorly responsible for keeping the oceans clean. These animals eat up edible waste and control pollution in the oceans. The lavas erupting at the bottoms of the oceans burn up the nearby non-biodegradable material. Thus, in natural conditions, oceans are self-cleaning systems.