13. The defect rate of a batch of products is 0.02. Drawing one item at a time with replacement from this batch, 100 times total, let $X$ denote the number of defective items drawn. Then $D X = $ ______
13. The defect rate of a batch of products is 0.02. Drawing one item at a time with replacement from this batch, 100 times total, let $X$ denote the number of defective items drawn. Then $D X = $ ______\\