It’s linear because it goes / increasing by the same rate. :)

If you only look at C values i.e. 30, 60, 120...etc it looks exponential

But the question asks for the growth of C with respect to w. 

As w increases by 1, C grows by 30. If you plot C on the y-axis and w on the x-axis, it'd be a line (hence linear growth). Another way to look at it is C/w = 30.