Answer 1

(a)

(b) Trenches, abyssal plains etc are useful for deep sea research. Trenches are the some of the most seismically zones which are responsible for causing earthquakes. Rock sediments from ocean floor have helped the scientists to determine its age which subsequently led to the development of plate tectonics theory.

(c) Islands are useful for protection of marine borders and naval-base building. For instance, India has a naval base at Andaman and Nicobar Islands.