设备的维修保养应达到哪些要求？
参考答案：
一般应达到整齐、清洁、润滑、安全四项要求。
Progress so far has been very good.____,we are sure that the project will be completed on time. [ ]
A. However
B. Otherwise
C. Therefore
D. Besides
南北朝民歌在内容和风格上有什么不同之处？
