Help please... what is the value of F(t)?

Question

An oscilloscope shows a wave that starts a cycle at its minimum of 20 units and reaches a maximum of 100 units before completing a five millisecond cycle. Which represents the relationship between time, t, and the value of the wave function, F(t)?

Answer 1

We could do this question from scratch but it would make sense to use the answers give as choices.

Remember -1 ≤ sin(x) ≤ 1 and -1 ≤ cos(x) ≤ 1. If 20 ≤ F(t) ≤ 80, then only the a. and c. remain. The problem also says F(0) = 20 which leaves only the a. The problem also gave the period as 5 ms and if time is in ms then only a. and b. have the right period. Any two of these reasons gives a. as the answer.