In the long run, a higher saving rate:
A. does not lead to a higher level of income because of deterioration in labor productivity
B. always leads to a higher level of productivity because of increasing returns to scale
C. does not always lead to a higher growth rate of output because of decreasing returns to capital
D. always leads to a higher growth rate of output because of improvement in the stock of capital
(Please give a brief expalantion of your answer.)