投资者进行备兑开仓的目的是（）。
A、锁定股票买入价格
B、增强持股收益，降低持股成本
C、降低股票卖出成本
D、锁定持股收益
参考答案：B
---Are you still working in Shanghai?
---No, I’m working in Hangzhou now. I ______ in Shanghai for three years.
A．worked
B．have worked
C．had worked
D．was working
何为混凝土的扩展度？
