When filing an income tax return, one can claim a deduction for charitable contributions. Let's simplify the income tax system and assume that the tax is proportional to the level of taxable income (income after deductions). (a) Suppose we increase the marginal income tax rate. An economic adviser claims that the effect of this tax change on the amount of charitable contributions is uncertain. Plot a simple graph that illustrates this situation (it involves the choice between charitable contributions and other spending), use it to explain why the effect is uncertain and explain what it depends on