Questions 13-18

　　In a benefit concert each of exactly seven solo

　　performers-Harris， Jones， McIntyre， Nelson， Strapp，

　　Trevino， and Williams-will sing. The concert director

　　is deciding the order in which the performers will sing

　　and is assigning each to exactly one of seven time slots，

　　numbered consecutively 1 through 7， according to the

　　following conditions：

　　Harris must sing at some time before McIntyre sings.

　　Strapp must sing at some time before Jones sings.

　　Trevino must sing either immediately before or

　　immediately after Nelson sings.

　　Williams must be assigned to time slot 3.

　　13.Which of the following could be the order， from first

　　through seventh， in which the performers sing？

　　（A）Harris， McIntyre， Williams， Trevino， Nelson，

　　Strapp， Jones

　　（B）Jones， Harris， Williams， Strapp， McIntyre，

　　Nelson， Trevino

　　（C）Strapp， McIntyre， Williams， Nelson， Trevino，

　　Harris， Jones

　　（D）Trevino， Harris， Williams， Strapp， Nelson， Jones，

　　McIntyre

　　（E）Trevino， Nelson， Harris， Strapp， Williams，

　　McIntyre， Jones

　　14.If there are exactly four performers who are to sing

　　after Nelson sings but before Strapp sings， Harris

　　must be assigned to time slot

　　（A）1

　　（B）2

　　（C）4

　　（D）5

　　（E）6

　　15.If Williams is to sing immediately after Harris sings

　　and immediately before Trevino sings， which of the

　　following performers could be assigned to time slot 6？

　　（A）Harris

　　（B）Jones

　　（C）Nelson

　　（D）Strapp

　　（E）Trevino