Here is what the LCDS Help file says about creating a Drag & Drop:
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For each column, enter items that belong to the category listed at the top of the column. You can have up to 9 total Category Items for the Drag and Drop game. Do not leave blank cells between populated cells within a row.
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The reason there are 15 spots in the LCDS template is so that you can have more items in one category than in another. For example, you may want 5 in one category and 2 each in the remaining categories.