All other things remaining equal, which of the following changes would cause the AD curve to become steeper? To become flatter?

a)(2 points) an increase in the interest-rate responsiveness of planned investment

b)(2 points) an increase in the interest-rate responsiveness of real money demand

c)(2 points) an increase in the marginal tax rate

d)(2 points) an increase in the marginal propensity to import

e)(2 points) an increase in the income responsiveness of real money demand