A. B, C, and D predict the items in the four drawers. A: There are no books in some drawers; B: There are books in all drawers; C: No pen in the second drawer; D: Third There are letters in each drawer. If only one of the four's conclusions is true, which of the following must be true? ['Pen in the second drawer.', 'Letters in the third drawer.', 'Books in four drawers.', 'No books in four drawers.']