完形填空。阅读下面短文，掌握其大意，从每题所给的A、B、C、D四个选项中，选出最佳选项。
One afternoon in January in 1998, Susan Sharp, 43, and her 8-year-old son David, were walking across an icy square, when Susan's cane(手杖) slipped on the ice. Her face 1 first into the mud. David 2 her mother's side, “Are you all right, Mom” 3 , Susan pulled herself up, “I'm okay, Honey,” she said.
Susan was falling more 4 since she had trouble walking. Every inch of ice was a 5 danger for her. “I wish I could do something,” the boy thought. David, too, was having 6 of his own. The boy had a speech problem, so at school he talked 7 .
One day, David's teacher announced a 8 homework. "Each of you is going to come up with an 9 ," she said. This was for "INVENT AMERICA", a national competition to encourage creativity in children.
An idea 10 David one evening. If only his mother's cane didn't slip on the ice. “What if I 11 your cane to a nail coming out of the bottom” he asked his mother.
“ 12 划破) floors,” Susan said. the sharp end would scratch(
