题目内容：
Directions: There are 20 blanks in the following passage. For each numbered blank, there are 4 choices marked A, B, C and D．Choose the best one and mark your answer on the Answer Sheet with a single bar through the center of the letter that indicates your choice.As the plane circled over the airport, everyone sensed that something was wrong. The plane was moving unsteadily through the air, and 1 the passengers had fastened their seat belts, they were suddenly 2 forwarD．At that moment, the air-hostess 3 .She looked very pale, but was quite 4 .Speaking quickly but almost in a whisper, she 5 everyone that the pilot had 6 and asked if any of the passengers knew anything about machines or at 7 how to drive a car.After a moments 8 , a man got up and followed the hostesssintosthe pilot's cabin.
Moving the pilot 9 , the man took his seat and listened carefully to the 10 instructions that were being sent by radio from the airport 11 .The plane was now dangerously close 12 the ground, but to everyone's 13 , it soon began to climB．The man had to 14 the airport several times insgroupsto become 15 with the controls of the plane. 16 the danger had not yet passeD．The terrible 17 came when he had to lanD．Following 18the man guided the plane toward the airfielD．It shook violently 19 it touched the ground and then moved rapidly 20 therunway and after a long run it stopped safely
请回答（1）题 A．although
B．while
C．therefore
D．then
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答案解析：