外汇储备作为国家货币的后盾，必须能够自由兑换——因此可认为是紧急情况下的流动性——而且比较稳定。
As a backstop, reserves need to be easily convertible (so they can be used as an emergency source of liquidity) and a good store of value.
盖茨说，向阿富汗移交安全控制工作将在这个国家比较稳定的地区开始，而且将在当地条件许可的情况下逐步进行。
Gates said the transition to Afghan control of security operations will begin in the country's more stable regions, and will be done gradually and as local conditions allow.
另一个问题是使用寿命的问题，利德泽说，硅材料光电板的使用寿命一般为20年左右，而且性能比较稳定。
Problems also remain with operational lifetime. Silicon devices will generally last around 20 years and are very stable, Lidzey says.
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