炉渣泡沫化的形成主要原因是（）。
A．炉渣中（FeO）含量高
B．炉渣温度高
C．炉渣表面张力低及小气泡产生
参考答案：C
---How long have you been here?
---Only about three minutes. Xiaoming and Mary _____here with me.[ ]
A. walk
B. have walked
C. had walked
D. walked
设森林F对应的二叉树为B，它有m个结点，B的根为P，p的右子树上的结点个数为n，森林F中第一棵树的结点个数是
A．m-n-1
B．n+1
C．m-n+1
D．m-n
