IX. Choose the correct option for each sentence

1. _____ aircraft were navigated visually.

a. Earliest b. The earliest c. Earlier d. Much early

2. Artificial horizons and turn-and-bank indicators allowed pilots to hold attitude and heading in clouds, _____ motivating the installation of navigation aids.

a. because b. nevertheless c. however d. hence

3. In the 1930s, aircraft were equipped with high-frequency direction finders _____ measured the bearing of broadcast stations relative to the axis of the aircraft.

a. than b. that c. who d. what

4. A fix was obtained _____ the direction toward two or more stations.

a. to plot b. plotting c. by plotting d. plotted

5. Forty years ago, it took five minutes _____ a fix manually on an on-board aeronautical chart.

a. to plot b. plotting c. by plotting d. plotted

6. Each satellite can cover millions of square miles of Earth, _____ a constellation of satellites can cover the entire Earth.

a. hence b. as c. when d. while

7. The introduction of Doppler and inertial navigators in the 1960s allowed drift _____ directly.

a. to observe b. to be observed c. be observed d. observe

8. The Doppler navigator measures the direction of the ground-speed vector _____ to the aircraft's centerline.

a. according b. due c. relative d. compared

9. VOR/DME and ILS _____ the basis of navigation in western countries ever since.

a. have been b. were c. are d. had been

10. During the 1960s, air-traffic controllers relied _____ surveillance radar in densely populated airspace.

a. to b. on c. by d. with

11. Outside the developed world, major cities and some airways had VOR/DME-based procedural traffic control, _____ aircraft flight plans could be separated from each other by human controllers.

a. moreover b. because c. as d. so