银首饰在空气中易与硫生成硫化银，故银首饰表面会呈（）。
参考答案：深褐色
I have warned him ______ that, but he didn't listen to my warning. [ ]
A. don't do
B. not to do
C. to not do
D. don't to do
园林中云梯是指（）。
A.以山石掇成的室外楼梯
B.如云的梯子
C.塑石如云状的梯级
D.群置石
