完形填空： 阅读下列短文，从所给的四个选项中，选出最佳答案

　Mrs Hill was very much interested in singing． She had a good voice（嗓子）, but some of her high notes（高音）seemed to sound like an axle（车轴） 　1　　　enough oil． Mrs Hill knew this very well, and tried her best to 　2 　them． As she didn't want to 　3 　the other 　4 　of the family, she usually went for long walks along the roads outside the city when she had time, and practised her high notes there．　5 　she 　6 　a car or person coming along the road, she stopped and 　7 　until she could 　8 　be heard before she started practising again 　9 　she was a shy（害羞的）woman．

　One afternoon a fast, open car came up 　10 　her so 　11 　and so fast that she did not hear it until it was only a few metres 　12 　her． She was singing some of her 　13 　and most difficult notes at the time, and as the car went past her, she saw a 　14 　look come over its driver's face． The driver put his brakes （闸）on, and as soon as the car stopped, he jumped out and began to 　15 　carefully at all his tyres（轮胎）to 　16 　if there was something wrong with 　17 　．Mrs Hill did not dare（敢）to 　18 　him what the 　19 　had really been, so he got back into his car and drove off as puzzled（迷惑不解）as he had been when he 　20 　．