伙委会值班时间是哪个时间段（）
A.11：00—12：00
B.11：20—12：20
C.11：40—12：30
D.12：00—12：30
参考答案：C
Which of the following is a characteristic of perfect competition（）
A. There are a few sellers.
B. The products of different firms are sold at different prices.
C. There are no barriers to entry into the market.
液压传动使用的密封材料应具备哪些性能？
