单项选择题 枇杷的叶子与古代乐器琵琶十分相似，故偕音而得“枇杷”之名。（ )

判断题 波速只决定于介质的性质。正确错误

单项选择题 ______ includes messages directed at retailers, wholesalers, and distributors, as well as industrial purchasers and professional such as lawyers and physicians.A.Directional advertisingB.Institutional advertisingC.Business-to-business advertisingD.Public service advertising