根据GB3469－83《文献类型与文献载体代码》规定,以单字母标识,J代表的参考文献的类型为:参考工具(如手册、年鉴)()

一列波，在第一种介质中的波长为λ1，在第二种介质中的波长为λ2，已知λ1 =3λ2，则该列波在这两种介质中频率之比和波速之比分别为（）。 点击查看答案

阅读理解：阅读下面的对话，选择合适的内容将对话补充完整。Dear Sirs,Account No.87561.{A;B;C;D;E}2.{A;B;C;D;E}3.{A;B;C;D;E}4.{A;B;C;D;E}5.{A;B;C;D;E}A、Your prompt attention to this matter would be greatly appreciated.If you have any questions regarding this account, please contact our office as soon as possible.B、Regards,LilyC、Please note that your account No.8756 was already a month overdue.As you are usually very prompt in settling your accounts, we wonder whether there is any special reason.D、If payment has recently been made, please accept our thanks and ignore this reminder.E、We think you may not have received the statement of account we sent you on August 30 showing the balance of US$ 80,000 you owe.We are sending you a copy and hope it may have your early attention. 点击查看答案