twice是不可数名词，例如：
1.We go there maybe once or twice a month.我们每月大概去那里一到两次。
2.You should visit your dentist at least twice a year.你应该每年至少去看两次牙科医生。
3.New drivers have twice as many accidents as experienced drivers.新驾驶员所出的事故是老驾驶员的两倍。
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