Hxen.com's tips：Every day persisted can learn English well!-->■Learn English Everyday■
crack – n. a very small space or opening between two things or two parts of something
rivet – n. a special kind of metal bolt or pin that is used to hold pieces of metal together
rush hour – n. a time during the day early in the morning or late in the afternoon when many people are traveling on roads to get to work or to get home from work
ultrasonic – adj. used to describe sounds that are too high for human beings to hear
