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.)