A、He was too old to see who did it. B、He was too kind to beat the boys. C、The boys ran too fast for him to catch. D、It was impos

There was once a man who spent all his time in his glasshouse. Flowers was his name, and flowers were his main joy in life. He grew flowers of every colour under the sun. He grew these flowers in order to enter them for competition. His greatest hope in life was to grow a rose of an entirely new color that would win him the silver cup for the Rose of the Year.
    Mr Flowers’ glasshouse was close to a public path, which was always used by children walking to and from school. Boys were often attracted to throw a stone or two at his glasshouse. So Mr Flowers did his best to be in or close by his glasshouse at the beginning and end of the school day.
    However, it was not convenient or possible to be on guard all the time. Mr Flowers had tried many ways to prevent harm to his glasshouse, but nothing that he had done had been successful.
    Then, just as he was giving up hope of ever winning the battle, and of growing the Rose of the Year, he had a truly wonderful idea. He put up a large notice made of good, strong wood, some metres away from the glasshouse, where it could be clearly seen from the path. He had painted on the board the words: DO NOT THROW STONES AT THIS NOTICE. After this, Mr Flowers had no further trouble. The boys were much more attracted to throw stones at the notice than at the glasshouse.
Questions 19 to 22 are based on the passage you have just heard.
19. What was Mr Flowers’ greatest hope?
20. Why did the boys often throw a stone or two at the glasshouse?
21. Why was Mr Flowers not able to prevent harm to his glasshouse at first?
22. Why did the boys throw stones at Mr Flowers’ notice?