Don't understand (取非method)

195. A proposed change to federal income tax laws would eliminate deductions from taxable income for donations a taxpayer has made to charitable and educational institutions. If this change were adopted, wealthy individuals would no longer be permitted such deductions. Therefore, many charitable and educational institutions would have to reduce services, and some would have to close their doors.
Premise: change to federal income tax laws->减少所得税对慈善教育机构的捐款->有钱人就不允许减少所得税
Conclusion: 机构减少服务
根据具体性和特殊性先排除选项（注意，要从推理的过程出发）

The argument above assumes which of the following?
(A) Without the incentives offered by federal income tax laws, at least some wealthy individuals would not donate as much money to charitable and educational institutions as they otherwise would have. （正确，取非3次，all would，他们捐一样多的钱，所以不用关）
(B) Money contributed by individuals who make their donations because of provisions in the federal tax laws provides the only source of funding for many charitable and educational institutions. （取非，provide other sources of funding for，那么在其他方面也可能会减少donation，所以不能否定结论）

i really don't understand what do CDer talked about  取非--- denial test ?

what is the use???   it seems that it is the same after the test