Directions: Read the given passage carefully and answer the questions that follow by selecting the most appropriate option. Helicopters are $\underline{very}$ different from airplanes. They can do three things that airplanes cannot do. First, when airplanes move upward, they must also move forward, but helicopters can move straight up without moving ahead. Second, helicopters can fly backward, which airplanes cannot do. Third, helicopters can use their rotors to $\underline{hover} in the air, which is impossible for airplanes. Helicopters can $\underline{perform}$ actions that airplanes cannot, they are used for different tasks. Since helicopters can take off without moving forward, they do not need a runway for takeoff. They are used in $\underline{congested}$ areas where there is no room for airplanes or in isolated areas, which do not have airports. Because they can hover, they are used on fire-fighting missions to drop water on fires. They are used in logging operations to lift trees out of forests. Helicopters are used as air ambulances to airlift patients out of situations, which are difficult to reach by conventional ambulances. The police use helicopters to follow suspects on the ground or to search for cars on the ground. Of course, helicopters have military uses because of their design and capabilities. The word 'congested' in the passage means