多用于加固山洞，作为石梁下面的垫梁。中国古典园林常用的方法是（）
A、银锭扣
B、铁爬钉
C、铁扁担
D、马蹄形吊架和叉形吊架
参考答案：C
The Smiths _____ their breakfast when the morning post came. [ ]
a. had
b. has been having
c. are having
d. were having
