---Is the Great Wall a wonderful place?   

---Yes,          tourists come here for a visit every year.

A．thousand of

B．thousands of

C．thousand

D．thousands

Brodkey的混合理论涉及三种基本扩散运动形式，下列哪种不属于此理论运动形式（）

A.分子扩散

B.涡旋扩散

C.体积扩散

D.层流扩散