当事人对合同变更的内容约定不明确的，推定为未变更。
参考答案：对
The train arrived and they were just to catch it. [ ]
A. on time; on time
B. on time; in time
C. in time; in time
D. in time; on time
矿井总回流中的瓦斯浓度达到（）时，必须查明原因，进行处理。
A.0.5%
B.0.75%
C.1%
