A four-person congressional committee is to be formed consisting of two senators and two representatives. The senators are to be chosen from W, X, Y, and Z; the representatives are to be chosen from P, Q, R, and S. The committee will be chosen by the vice-president subject to the following conditions:

a. S will not serve on the committee unless X does.
b. Y will not serve on a committee with Q.
c. The vice-president has decided that Y or Z must be included, but neither Y nor Z is willing to serve with the other. 25.
d. If the vice-president chooses S to be on the committee, which of the following CANNOT be on the committee