What Is the First Step in Interpolating the Elevation of the Tree

What is the first step in interpolating the elevation of the tree at point A in the accompanying figure?

A) Subtract an amount from the nearest contour line
B) Scale the distance between the two adjacent contour lines
C) Scale the distance from the unknown point to the nearest contour line
D) Multiply the contour interval by the fraction of the distance between the contour lines