为了确保持续保护，要不时地添加防腐剂。
答案解析
解析：
### 题干解析
题干提到“为了确保持续保护，要不时地添加防腐剂。”这句话的意思是，为了保持某种保护效果（可能是防止金属腐蚀），需要定期添加防腐剂。这里的“持续保护”强调了防腐剂的必要性和定期使用的重要性。
### 选项分析
- **A: Corrosion inhibitor is necessary from time to time to ensure continued protection.**
- 这个选项的意思是“防腐剂是必要的，需要不时地添加，以确保持续的保护。”这个表述与题干的意思完全一致，强调了定期添加防腐剂的重要性。
- **B: Corrosion inhibitor is necessary frequently so that protection remains effective.**
- 这个选项的意思是“防腐剂是必要的，需要频繁地添加，以保持保护的有效性。”虽然这个选项也提到了添加防腐剂的重要性，但“频繁”这个词可能暗示着比“从时到时”更高的频率，可能会导致误解。
- **C: Corrosion protection is necessary but is ineffective for cost reasons.**
- 这个选项的意思是“防腐保护是必要的，但由于成本原因是无效的。”这个表述与题干的意思完全相反，强调了防腐保护的无效性，因此不符合题干的要求。
### 正确答案
因此，正确答案是 **A**，因为它准确地反映了题干的意思，强调了定期添加防腐剂以确保持续保护的重要性。
### 深入理解
为了帮助你更好地理解这个知识点，我们可以用一个生动的例子来说明。
想象一下，你有一辆汽车。为了保持汽车的良好状态，你需要定期进行保养，比如更换机油、检查刹车系统等。如果你忽视这些保养，汽车的性能可能会下降，甚至出现故障。同样，防腐剂在金属保护中起到的作用就像汽车保养一样，定期添加防腐剂可以确保金属表面不被腐蚀，保持其性能和外观。
### 总结
相关知识点：
添加防腐剂确保保护考点
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