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.

The invention of the airplane, like many great inventions, was initially met with ridicule and disbelief.

Although many people who heard about the first powered flight on December 17, 1903, were excited and impressed, others reacted with peals of laughter. The idea of flying an aircraft was repulsive to some people. Although Orville and Wilbur Wright, the inventors of the first flying machine, were called impulsive fools, the Wrights continued their experiments in aviation.

Orville and Wilbur Wright had always had a compelling interest in aeronautics and mechanics. As young boys they earned money by making and selling kites and mechanical toys. Later, they designed a newspaper-folding machine, built a printing press, and operated a bicycle-repair shop. In 1896, when they read about the death of Otto Lilienthal, the brother's interest in flight grew into a compulsion.

Lilienthal, a pioneer in hang-gliding, had controlled his gliders by shifting his body in the desired direction. This idea was repellent to the Wright brothers, however, and they searched for more efficient methods to control the balance of airborne vehicles. In 1900 and 1901, the Wrights tested numerous gliders and developed control techniques. The brothers' inability to obtain enough lift power for the gliders almost led them to abandon their efforts.

After studying and experimenting with model wings in a wind tunnel, the Wright brothers designed their first airplane in 1903 for less than one thousand dollars. They even designed and built their own lightweight gasoline engine. On December 17, they successfully flew the plane for twelve seconds and 120 feet, despite a turbulent start.

By 1905 the Wrights had perfected the first airplane that could turn, circle, and remain airborne for half an hour at a time. Others had flown in balloons or in hang gliders, but the Wright brothers were the first to build a full-size machine that could fly under its own power. As the contributors of one of the most outstanding engineering achievements in history, the Wright brothers are accurately called the fathers of aviation.

The underlined word ‘‘which” in the passage refers to_______. A. a common element