秦汉______的建造，无疑是汉民族形成的重要标志。
参考答案：长城
The companies are working together to create _____ they hope will be the best means of transport in the 21st century.
A．which
B．that
C．what
D．who
脂质体在体内的作用机制错误的是（）
A.吸附
B.脂交换
C.内吞
D.溶解
E.融合
