Economists Often Treat the Economy's Capital Stock as Fixed Because

Economists often treat the economy's capital stock as fixed because

A) labour is a more important factor of production than capital,so economists ignore capital.
B) it takes a long time for new investment and the scrapping of old capital to affect the overall quantity of capital.
C) there is very little capital in the economy compared to the amount of labour.
D) unless the interest rate changes,the capital stock doesn't change.