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 from 36 to 42.

Hydrogen is the most common element in the universe and was perhaps the first to form. It is among the ten most common elements on Earth as well and one of the most useful for industrial purposes. Under normal conditions of temperature, hydrogen is a gas. Designated as H, hydrogen is the first element in the periodic table because it contains only one proton. Hydrogen can combine with a large number of other elements, forming more compounds than any of the others. Pure hydrogen seldom occurs naturally, but it exists in most organic compounds, that is, compounds that contain carbon, which account for a very large number of compounds. Moreover, hydrogen is found in inorganic compounds. For example, when hydrogen burns in the presence of oxygen, it forms water.

The lightest and simplest of the elements, hydrogen has several properties that make it valuable for many industries. It releases more heat per unit of weight than any other fuel. In rocket engines, tons of hydrogen and oxygen are burned, and hydrogen is used with oxygen for welding torches that produce temperatures as high as 4,000 degrees F and can be used in cutting steel. Fuel cells to generate electricity operate on hydrogen and oxygen.

Hydrogen also serves to prevent metals from tarnishing during beat treatments by removing the oxygen from them. Although it would be difficult to remove the oxygen by itself, hydrogen readily combines with oxygen to form water, which can be heated to steam and easily removed. Furthermore, hydrogen is one of the coolest refrigerants. It does not become a liquid until it reaches temperatures of-425 degrees F. Pure hydrogen gas is used in large electric generators to cool the coils.

Future uses of hydrogen include fuel for cars, boats, planes, and other forms of transportation that currently require petroleum products. These fuels would be lighter, a distinct advantage in the aerospace industry, and they would also be cleaner, thereby reducing pollution in the atmosphere.

Hydrogen is also useful in the food industry for a process known as hydrogenation. Products such as margarine and cooking oils are changed from liquids to semisolids by combining hydrogen with their molecules. Soap manufacturers also use hydrogen for this purpose. In addition, in the chemical industry, hydrogen is used to produce ammonia, gasoline, methyl alcohol, an many other important products.