The Accompanying Graph Shows the Cost Curves for Moe's Mushroom

The accompanying graph shows the cost curves for Moe's mushroom gathering business, which is perfectly competitive. The curve labeled A is upward sloping because:

A) of high fixed costs.
B) of diminishing returns to Moe's variable factors of production.
C) an increase in the demand for mushrooms increases the supply of mushrooms.
D) variable costs increase as output increases.