the possibility (a) is possible. here is an example : take the set $ S=[2,2,2,2,1,1,...(1484 ones)] $. let us assume the set is $ [a_1,a_2,...a_n] $ by fermat's little theorem we have $ 7|({a_1}^7-a_1)+({a_2}^7-a_2)+...({a_n}^7-a_n)=({a_1}^7+{a_2}^7...+{a_n}^7)-(a_1+a_2+...a_n)=1998-1492=506 $, which is not possible, so it rules out possibility (b)