单项选择题 For a frequency distribution, the class midpoint is:

单项选择题 收银六步骤的排序是正确的①欢迎顾客②确认点餐内容③点餐④收银找零⑤建议性销售⑥呈递收银条指引顾客（）

未知题型 Please convert active voice into passive voice in the following sentence. We should attach importance to environmental protection.