Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Man is a land animal, but he also closely tied to the sea. Throughout history the sea has served the needs of man. The sea has provided man with food and convenient way to travel to many parts of the world. Today, nearly two thirds of the world's population lives within 80 kilometers of the sea coast.

In the modern technological world, the sea offers many resources to help mankind survive. Resources on land are beginning to be used up. The sea, however, still can be hoped to supply many man's needs. The list of riches of the sea yet to be developed by man's technology is impressive. Oil and gas explorations have been carried out for nearly 30 years. Valuable amounts of minerals existing on the ocean floor are ready to be mined.

Fish farming promises to be a good way to produce large quantities of food. The culture of fish and shellfish is an ancient skill practiced in the past mainly by Oriental people.

Besides oil and gas, the sea may offer new sources of energy. Experts believe that the warm temperature of the ocean can be used in a way similar to the steam in a steamship. Ocean currents and waves offer possible use as a source of energy.

Technology is enabling man to explore ever more deeply under the sea. The development of strong, new materials has made this possible. The technology to harvest the sea continues to improve. Experts believe that by the year 2020 the problems that prevent US from exploiting fully the food, minerals, and energy source of the sea will be largely solved.

Smoke, dirt and noise are all kinds of __________.