The Short-Run Supply Curve for a Competitive Industry Is Upward-Sloping

Multiple Choice

The short-run supply curve for a competitive industry is upward-sloping because:

A) firms must pay more for inputs as more are hired./firms must incur higher costs the more inputs they hire.
B) the efficiency of the variable inputs decreases as more such inputs are employed in production.
C) new firms enter the industry as product prices increase.
D) of the law of diminishing marginal utility.