This boulder is volcanic in origin and yet the rest of the rock in this area is sedimentary. Since this area was covered by southward-moving glaciers during the last ice age, this boulder was probably deposited here, hundreds of miles from its geological birthplace, by a glacier. Which one of the following, if true, most seriously undermines the conclusion drawn in the argument above? ['Most boulders that have been moved by glaciers have not been moved more than 100 miles.', 'The closest geological source of volcanic rock is 50 miles north of this boulder.', 'The closest geological source of volcanic rock is 50 miles south of this boulder.', 'There are no geological sources of volcanic rock north of this boulder.']