If S=∑r=0m n+rCk, then
S+nCk+1=n+mCk+1
S+nCk+1=n+m+1Ck+1
S+nCk=n+mCk
None of these
S=∑r=0m n+rCk=nCk+n+1Ck+n+2Ck+…+n+mCk=nCk+1+nCk+n+1Ck+n+2Ck+…+n+mCk−nCk+1=n+1Ck+1+n+1Ck+n+2Ck+…+n+mCk−nCk+1∴ S=n+m+1Ck+1−nCk+1