Graphs of Sine and Cosine · Medium problem

$-15\cos\!\left(\tfrac{\pi}{15}t\right)+18$

$-15\cos\!\left(\tfrac{\pi}{15}t\right)-18$

$-15\cos\!\left(\tfrac{\pi}{15}t\right)+19$

$-15\cos\!\left(\tfrac{15}{\pi}t\right)+18$