问题如下：
Suppose the weighted average cost of capital of the Gadget Company is 10 percent. If Gadget has a capital structure of 50 percent debt and 50 percent equity, a before-tax cost of debt of 5 percent, and a marginal tax rate of 20 percent, then its cost of equity capital is closest to:
选项：
A. 12 percent.
B. 14 percent.
C. 16 percent.
解释：
C is correct.
Using Equation 1 from the reading:
0.10 = (0.50)(0.05)(1 – 0.20) + (0.50)
= 0.16 or 16 percent
请问边际税率是什么意思呢？