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
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，
（B）Jones， Harris， Williams， Strapp， McIntyre，
（C）Strapp， McIntyre， Williams， Nelson， Trevino，
（D）Trevino， Harris， Williams， Strapp， Nelson， Jones，
（E）Trevino， Nelson， Harris， Strapp， Williams，
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
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？