完形填空：共l5小题，通读下面的短文。掌握其大意。然后，从每小题的四个选择项中选出可填入相应空白处的最佳选项。
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.
1. 通读以上的短文，请选择可填入___21___的最佳选项。
ABefore
BWhen
CSince
DThough
