As a general rule of thumb, children this age should not spend more than one hour on homework.照理说，这个年龄的儿童家庭作业不应超过一个小时。A school dental service exists in theory, but in practice, there are few dentists to work in them.照理说，学校也有牙医诊所，可实际上，里面的牙医寥寥无几。Theoretically, the price is supposed to be marked on the shelf.照理说，价格应该标在货架上。