两个频率相同的正弦交流电的初相位之差，称为（）。
无机盐对于机体生理有何作用？
A customer is asking for System x server quote. The customer provided IBM with a complete specification except for the backup device. What should a sales specialist do to position a total solution for the customer？（） ["Do not quote any additional services.","Add a tape device as an option to the quote. ","Ask for a special bid to lower the total price.","Just quote exactly to the customer’s specification."]
龙滩水电工程工程主要由（）、地下引水发电系统和通航建筑物三大部分组成，以发电为主，并有防洪、航运效益。 ["溢洪道","大坝","排水廊道","溢流坝"]
影响儿童游戏的环境因素，从性质上可分为（） ["心理和物质环境","物理和社会环境","生态和社会环境","物理和心理环境"]
产业集群
