The lines with equations ax−2y=c and 2x+by=−c are perpendicular and intersect at (1,−5). What is c?<spanclass=′latex−bold′>(A)</span>−13<spanclass=′latex−bold′>(B)</span>−8<spanclass=′latex−bold′>(C)</span>2<spanclass=′latex−bold′>(D)</span>8<spanclass=′latex−bold′>(E)</span>13