Final answer:
Paper changes when it absorbs water because the water molecules form hydrogen bonds with hydroxyl groups in the cellulose fibers, causing the fibers to expand and the paper to become bulkier.
Step-by-step explanation:
When paper absorbs water, the cellulose fibers within it expand rather than contract. This is because cellulose, which makes up both paper and cotton, contains many -OH (hydroxyl) groups. When the paper contacts water, the water molecules form hydrogen bonds with these hydroxyl groups. This interaction draws the water molecules up the cellulose fibers, causing them to swell due to the adhesion between the water and the fibers. Moreover, the water absorbed contributes to the volume of the cellulose fibers, making the paper bulkier and less rigid. Therefore, the correct response to why paper changes so much when it absorbs water is B) The cellulose fibers expand.