风口各套由于所处工作条件不同，采用了不同的材质，大套一般用铸钢制造，中套用（）制作，小套用铜制作。
参考答案：铜
计算：20=（ ）。
I _____his letter for a week. [ ]
A. heard from
B. have received
C. have had
D. have got
