根据句意和首字母提示补全单词。 1. You had better brush your t________ twice a day. 2. You can i________ your English if you study hard. 3. The good news s________ very fast. We all know it now. 4. Walking is a good exercise and it is n________ for your health. 5. The classroom is so noisy that we can't hear the teacher c________.