操作系统是一种系统软件，它统一管理计算机的所有硬件资源和软件资源。
参考答案：对
若等腰三角形的边长分别为4和6，则它的周长为（ ）。
—________ you called yesterday evening?
—My sister. Why?[ ]
A. Who it is that
B. Who is it that
C. Who was it that
D. Who it was that
