（）缺陷的特征是带钢表面有周期性的凹坑。
A.划伤
B.擦伤
C.压印
参考答案：C
At first they talked in English but ended up _____ in Chinese. [ ]
A. speak
B. to speak
C. speaking
瓦斯保护可以保护何种故障？
