You have recently become interested in collecting used classic vinyl recordings. The jackets for these disks often have no liners to protect the records from dust and other sources of scratches, so you seek to purchase some liners from online vendors. Two web sites offer 12-in. inner plastic liners: site A advertises liners with a thickness of 50 microns, while the liners at site B are 3 mils thick. Which of the sites is selling the thicker liner?