Often, the fact ________ unemployed people get help from government ________ this from happening, but not always.

A．how, to prevent

B．that, prevents

C． how, preventing

D．why, prevented

下列哪一项不是X射线衍射的方法（）

A.劳厄法

B.粉晶法

C.德拜法

D.转晶法