Part A
The original signal shown in the first plot is y(t) = sin(t) with a period of $ 2\pi $
The plot on the left shows the same signal but in DT (y[n] = sin(n)). The sampling frequency is at $ \pi/2 $ (y[n] = sin($ \pi/2 $*n)). The signal repeats itself every 4 sec so that y[n] = y[n+4].
