Optimism about vaccine roll-out leads to surge in business investment." Taken by itself, A. both the consumption and the saving function shift up; hence, a shift of the AE function and a change in equilibrium national income are unpredictable. B. the consumption function shifts up, the AE function shifts up, and equilibrium national income increases. C. the investment function shifts up, the AE function shifts up, and equilibrium national income increases. D. the investment function shifts up and the saving function shifts down; hence, a shift of the AE function and a change in equilibrium national income are unpredictable