1.abbr. describing something that can be left out isn't required

　　(a) I.P.O.

　　(b) PIN

　　(c) etc.

　　(d) N/A

　　2.abbr. numeral; mathematical value or its symbol; unit in a series; part

　　(a) PR

　　(b) no.

　　(c) Ph.D.

　　(d) plc

　　3.abbr. legally established as a corporation

　　(a) A.O.B.

　　(b) Inc.

　　(c) C.F.O.

　　(d) Ltd.

　　4.abbr. cash payment to be made upon receipt of products or goods

　　(a) AOL

　　(b) C.O.D.

　　(c) I.O.U.

　　(d) G.N.P.

　　5.abbr. firm, company that is organized in such a way as to give its owners limited liability

　　(a) Ltd.

　　(b) hr

　　(c) ASAP

　　(d) plc

　　正确答案：dbbba