One Reason That Variable Factors of Production Tend to Show

One reason that variable factors of production tend to show diminishing returns in the short run is that:

A) capital equipment is often idle in the short run.
B) there is only so much that can be produced using additional variable inputs when some factors of production are fixed.
C) large firms cannot effectively manage their resources.
D) the cost of employing additional resources increases as firms employ more of those resources.