Manufacturers of writing paper need to add mineral "filler" to paper pulp if the paper made from the pulp is to look white. Without such filler, paper products look grayish. To make writing paper that looks white from recycled paper requires more filler than is required to make such paper from other sources. Ttherefore, barring the more efficient use of fillers in paper manufacturing or the development of paper-whitening technologies that do not require mineral fillers, if writing paper made from recycled paper comes to replace other types of writing paper, paper manufacturers will have to use more filler than they now use. Which one of the following is an assumption on which the argument depends? ['The total amount of writing paper manufactured worldwide will increase significantly in the future.', 'Certain kinds of paper cannot be manufactured from recycled paper.', 'Beyond a certain limit, increasing the amount of filler added to paper pulp does not increase the whiteness of the paper made from the pulp.', 'Grayish writing paper will not be a universally acceptable alternative to white writing paper.']