2 y(t) 之拉氏轉換（Laplace transform）為  ，其中 u(t) 為單位步階（unit step）函數，則 y (t) = ？ 
(A)y(t)=(1-u(t-2π))sin 3t
(B) y(t)=3(1-u(t-2π))cos3t
(C) y(t)=3cos3(t-2π) 
(D)y(t)=u(t-2π)sin3t