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Đáp án D thành “who was recommeded’
Giải thích: Đằng sau ta thấy có by + O nên đây là câu bị động.
Choose from A, B, C, or D the one that best answers each of the questions.
A pilot cannot fly by sight alone. In many conditions, such as flying at night and landing in dense fog, a pilot must use radar, an alternative way of navigating. Since human eyes are not very good at determining speeds of approaching objects, radar can show a pilot how fast nearby planes are moving. The basic principle of radar is exemplified by what happens when one shouts in a cave. The echo of the sounds against the walls helps a person determine the size of the cave. With radar, however, the waves are radio waves instead of sound waves. Radio waves travel at the speed of light, about 300,000 kilometers in one second. A radar set sends out a short burst of radio waves. Then it receives the echoes produced when the waves bounce off objects. By determining the time it takes for the echoes to return to the radar set, a trained technician can determine the distance between the radar set and other objects. The word “radar”, in fact, gets its name from the term “radio detection and ranging”. “Ranging” is the term for detection of the distance between an object and the radar set. Besides being of critical importance to pilots, radar is essential for air traffic control, tracking ships at sea, and for tracking weather systems and storms.
51. What is the main topic of this passage?
A. The nature of radar. B. History of radar. C. Alternatives to radar. D. Types of ranging.
52. According to the passage, what can radar detect besides location of objects?
A. Shape B. Size C. Speed D. Weight
53. The word “exemplified” in the passage can be replaced by_________.
A. “specified” B. “resembled” C. “illustrated” D. “justified”
54. The word “shouts” in the passage most closely means_________.
A. “exclaims” B. “yells” C. “shoots D. “whispers”
55. Which of the following words best describes the tone of this passage?
A. argumentative B. explanatory C. humorous D. imaginative
56. According to the passage, the distance between a radar set and an object can be determined by_________.
A. the time it takes for a burst of radio waves to produce echoes when the waves bounce off the object
B. the term “ranging” used for detection of the distance between an object and the radar set
C. the time it takes for the radio waves to produce echoes and bounce off the object
D. the time it takes for the echoes produced by the radio waves to return to the radar set
57. Which type of waves does radar use?
A. tidal B. sound C. heat D. radio
58. The word “tracking” in the passage most closely means_________.
A. sending B. searching for C. ranging D. repairing
59. Which of the following would most likely be the topic of the next paragraph?
A. A history of flying. B. Other uses of radar. C. The technology used by pilots. D. Uses of some technology.
60. What might be inferred about radar?
A. It takes the place of a radio. B. It has improved navigational safety. C. It was developed from a study of sound waves. D. It gave birth to the invention of the airplane.
Làm bài này rối não quá 
#Yumi
Đáp án A => who/ that
Đây là mệnh đề quan hệ thuộc về the person nên ta không dùng đại từ which mà phải dùng who/ that