根据税收征收管理法律制度的规定，下列发票中，属于专用发票的是（）。
A.机动车销售统一发票
B.商品房销售发票
C.商业零售发票
D.铁路客票
参考答案：D
My American friend Sam is interested in Yangzhou very much. He wonders _______.
A．that Yangzhou was very rich in the ancient times
B．who built Ge Garden and He Garden
C．when was the Wenchang Pavilion built
D．what is the City Flower of Yangzhou
管路系统的特性曲线是随着（）的增加而上升。
A、流量
B、扬程
C、轴功率
D、效率
