Twenty professional income-tax advisors were given identical records from which to prepare an income-tax return. The advisors were not aware that they were dealing with fictitious records compiled by a financial magazine. No two of the completed tax returns agreed with each other, and only one was technically correct. If the information above is correct, which one of the following conclusions can be properly drawn on the basis of it? ['People are more likely to have an incorrectly prepared tax return if they prepare their own tax returns than if they hire a professional income-tax advisor.', 'All professional income-tax advisors make mistakes on at least some of the tax returns they prepare.', 'In order to insure that tax returns are correct, it is necessary to hire professional income-tax advisors to prepare them.', 'The fact that a tax return has been prepared by a professional income-tax advisor provides no guarantee that the tax return has been correctly prepared.']