题目内容：
通读下面的短文，回答题。 As a boy I dreamed of flying airplanes． 21 I told Dad I wanted to be a pilot,he said, "Youneed to do your job well on the farm first, and 22 you may realize your dream． "The farm was aneveryday responsibility (职责)--that． 23 Saturday, Sunday and Christmas too． Every morning Igot up at four o' clock to milk and 24 the cows． After the milk was bottled, my older brothers andI 25 it to people' s homes． I was paid five cents a bottle． At that time I 26 that to be success-ful you have to take care of your responsibilities．
When I was 16 ,I signed up for flying lessons． To pay for the lessons I 27 a small restaurantwith some friends． One of our regular 28 was the president of the Farmers Bank． Every 29 Igot,I talked to him about my dream． One day I told him I 30 $3,500 to buy my own 31 ． Helent me the money so that I could fly people within 150 miles of the airport．
Twelve years later I was flying people around the world 32 a pilot of a big airline company．
After ten years in the company I started my own 33 ．
Today I run a billion-dollar company, but it was on the farm that I learned the principles(原则)that have 34 me． Hard work and determination do 35 dreams come true．
________ A．Before
B．When
C．Since
D．Though
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