A student is measuring the length of an icicle, y, every hour, x. The icicle is currently 13 inches long and is melting at a rate of 0.4 inches per hour. Find and interpret the slope for this relationship.

0.4; for every additional hour, the length of the icicle increases by 0.4 inches
−0.4; for every additional hour, the length of the icicle decreases by 0.4 inches
13; the length of the icicle when the student first measures it
−13; the length of the icicle when the student first measures it