The Following Figure Shows Two Isocost Lines $I S _ { 1 }$

Multiple Choice

The following figure shows two isocost lines ( $I S _ { 1 }$ and $I S _ { 2 }$ ) and two isoquants for the production of 200 and 350 tons of corn.The axes show the quantities of the two types of fertilizers used in corn production.

-Refer to Figure .An increase in the slope of an isocost implies:

A) the firm is using more of the input measured on the X-axis.
B) the total cost incurred by the firm has declined.
C) the input measured on the X-axis has become costlier.
D) the price of both inputs has declined.