Please help me with Word Function problem

An airplane is flying at a speed of 100 mi/h at an altitude of one mile and passes directly over a radar station at time t = 0.

(a) Express the horizontal distance d (in miles) that the plane has flown as a function of t.

d = 100t (I came up with this answer and it is correct)

(b) Express the distance s between the plane and the radar station as a function of d.

s = ???

(c) Use composition to express s as a function of t.

s = ???

Please help me with the problems b and c. I don't understand what they are talking about. Like for d shouldn't the answer be 100d but the computer said it was incorrect. Please show your steps and explain how to solve these two problems (b) and (c). Thank you so much!