填空题
It may be necessary to stop（）(每隔一定时间) in the learning process and go back to the difficult points in the lessons.
at intervals
填空题 There’s a man at the reception desk who seems very angry and I think he means（）(想找麻烦).
填空题 We’d like （）(预订一张餐桌) five for dinner this evening.
填空题 The price of beer（）(从50美分到4美元不等) per liter during the summer season.
