Scientists study the ocean in many different ways. Although many marine  scientists focus on animals in the ocean, others seek to gain a better understanding of the ocean's chemical and physical components  . Scientists are learning a great deal about the natural and human-caused processes that shape the ocean and its many life forms. Much of the present research focuses on how human activity influences the ocean. Clean water is a limited resource, and ocean pollution does harm to plant and animal health. Some scientists are now studying ocean water to find out just how contaminated it is. Scientists are also working on new methods to estimate the presence of poisons in the ocean, which will help them develop better techniques for cleaning up the water. Others look at how sea living things get used to poisons because poisonous sea life has an effect on humans too. For example, some seafood is poisoned, which may cause a threat to humans who eat it. Another type of marine scientist tries to learn more about the history of the ocean. Understanding the ocean's past tells scientists more about its patterns and cycles today and in the future. The ocean's past, which is recorded by deposits   in the ocean floor, helps scientists understand such important matters as global climate change and other environmental differences. The more researchers learn about the history of the ocean and its relation to climatic and environmental changes, the more likely that they will one day understand what causes such changes. Other scientists are concerned with understanding how ocean systems work. The word systems refers to all the conditions and elements that are continually changing the ocean. Understanding these systems is necessary to protect the ocean and ensure the safety of its life forms. The cycles of ocean life also have an enormous effect on human life; for example, studying coastal waters is important because they provide much of the fish that humans eat. Marine geologists are earth scientists who work along the ocean floor. The Ocean Drilling Program, a 20-nation effort, drills the ocean floor and removes material for study. Among other achievements, this program has found evidence of a giant meteorite impact  ,which supports the theory that dinosaurs died out after the earth suffered a climate change caused by a crash with a large meteor. Other marine geologists focus on more recent events, such as the destruction of shorelines. According to the passage, we can know that one focus of marine geology is   _  .
the destruction of the ocean plants the form of the ocean floor the development of ocean living things the history of the ocean