单项选择题

As a boy I dreamed of flying airplanes.  21  I told Dad I wanted to be a pilot, he said, "You need to do your job well on the farm first, and  22  you may realize your dream." The farm was an everyday responsibility(职责)-that  23  Saturday, Sunday and Christmas too. Every morning I got up at four o' clock to milk and  24  the cows. After the milk was bottled, my older brothers and I  25  it to people' s homes. I was paid five cents a bottle. At that time I  26  that to be successful 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 restaurant with some friends. One of our regular  28  was the president of the Farmers Bank. Every  29  I got, l talked to him about my dream. One day I told him I  30  $3, 500 to buy my own  31 . He lent 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