Whitney currently contributes $5,000 a year to her retirement account and is thinking about increasing the amount to $15,000 a year because her tax rate would decrease from 24 percent to 22 percent, which would save a substantial amount in taxes. She will be able to apply the savings to her childrens college funds. What is this an example of?

a. Marginal reasoning

b. Sensitivity analysis

c. Opportunity cost

d. Realistic assumptions