A ladder is formed by removing some consecutive unit squares of a 10×10 chessboard such that for each k−th row (k∈{1,2,…,10}), the leftmost k−1 unit squares are removed. How many rectangles formed by composition of unit squares does the ladder have?<spanclass=′latex−bold′>(A)</span>625<spanclass=′latex−bold′>(B)</span>715<spanclass=′latex−bold′>(C)</span>1024<spanclass=′latex−bold′>(D)</span>1512<spanclass=′latex−bold′>(E)</span>None of the preceding