(a) S1={abcd∈{G,B}4:c=B} (b) S2={abcd∈{G,B}4:a=b=c=d} (c) S3={abcd∈{G,B}4:abcd≠GGGG} (d) S4={a1a2a3a4∈{G,B}4:ai≠ai+1,i∈{1,2,3}}