问题如下：
Corporate bond secondary market trading most often occurs:
选项：
A.on a book-entry basis.
B.on organized exchanges.
C.prior to settlement at T + 1.
解释：
A is correct.
The vast majority of corporate bonds are traded in over-the-counter (OTC) markets that use electronic trading platforms through which users submit buy and sell orders. Settlement of trades in the OTC markets occurs by means of a simultaneous exchange of bonds for cash on the books of the clearing system "on a paperless, computerized book-entry basis."
b为什么不对？不是统一组织交易么
