Palmer Co. had a deferred tax liability balance due to a temporary difference at the beginning of 2014 related to $600,000 of excess depreciation. In December of 2014, a new income tax act is signed into law that lowers the corporate rate from 40% to 35%, effective January 1, 2016. If taxable amounts related to the temporary difference are scheduled to be reversed by $300,000 for both 2015 and 2016, Palmer should increase or decrease deferred tax liability by what amount?

a. Decrease by $30,000

b. Decrease by $15,000

c. Increase by $15,000

d. Increase by $30,000

why b ??