轧制时，变形区内无论是前滑区还是后滑区，轧件所受的摩擦力方向都指向中性向。
参考答案：对
简述亚甲蓝比色法测定阴离子合成洗涤剂的原理。
It was the first time Mary _____ such a big opening ceremony.
A．has attended
B．attended
C．had attended
D．should attend
