"It" is a pronoun. As a personal pronoun it refers back to something that has already been mentioned.
是代词，在用作人称代词时指代前文提过的某事。
As has already been mentioned, they were supported by an airlift of hope, and we are so honored to be joined by Colonel Halvorsen, 92 years old -- the original "candy bomber. ""
正如刚才提到的，柏林人得到了承载着希望的空运支持，而我们很荣幸92岁高龄的哈尔沃森上校今天来到这里—— 他是当年的“糖果投弹手”。
One such model has been already mentioned in this article: the COCOMO II equation that links effort (cost) to personnel, environment, quality, size, and process.
在本文中我们已经提到了一个模型：COCOMOII等式，它把工作量(成本)与人员，环境，质量，规模和过程联系起来。
If the version of PAC mentioned by the identifier has already been cached by the PLAM-ed server, the session is set up and no PACs are required to be pulled in from the initiating host.
如果标识符中使用的PAC版本早已在采用PLAM的服务器上进行缓存了，那么就会直接建立这个会话，而不需要从发起主机上检索PAC。
This thought, mentioned in psychological research already, has not been paid attention to in the research of art thought.
这种思维在心理学的研究中已经被人提到，但是在艺术思维的研究中，还不为人所重视。
The College Board Management Decision Information System thus designed and implemented for the above-mentioned activities has already been running well in practice.
基于上述工作设计完成的“高校董事会管理决策信息系统”已在本人所工作的单位试用，反映较好。
The computer has been at present already pretty universal, computer knowledge studying has already mentioned hitherto unknown altitude.
目前计算机已经相当普及，计算机知识的学习已经提到了前所未有的高度。
The computer has been at present already pretty universal, computer knowledge studying has already mentioned hitherto unknown altitude.
目前计算机已经相当普及，计算机知识的学习已经提到了前所未有的高度。
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