Which of the following statements best describes the property of the system T if y1(t)=T[x1(t)], y2(t)= T[x2(t)] implies T[ax1(t-s)+bx2(t-s)]= ay1(t-s)+by2(t-s), .A:the system is linear B:it is a linear time-invariant system C:the system satisfies the principle of superposition D:the system satisfies additivity and time-invariance 答案: it is a linear time-invariant system