Answer: y = cos(2x) which is choice C
Step-by-step explanation:
Normally, y = cos(x) has a period of 2pi. This means that every 2pi horizontal units, the graph repeats itself. However, we can see that the period here is pi units instead.
One way to see this is to start at (0,1). This is a local max point. Move to its neighboring local max at (pi,1). We have moved pi units along the x axis and the cycle is finished, after which point the cycle repeats itself.
Since T = pi is the period, we then can say B = 2pi/T = 2pi/pi = 2
This is then plugged into y = A*cos(B(x-C))+D where
A = 1
C = 0
D = 0
That leads us to y = cos(2x)
Since the period is often connected to time values, it might help to think of this wave's oscillations occurring twice as often compared to y = cos(x). So that might help see why we replace the x with 2x.