下列不属于铸件中常见缺陷的是（）
A、缩孔
B、疏松
C、成分偏析
D、折叠
参考答案：D
命题“如果x＞2且y＞2，那么x+y＞4”的否命题是______．
In 1901, H. G Wells, an English writer, wrote a book_______ a trip to the moon. [ ]
A. describe
B. described
C. describing
D. to describe
