防止高压窜入低压的安全措施是什么？简述它的动作原理。
有价证券与无价证券最为明显的区别是它的流通性。
（）称为铅垂线。
1997年3月修订的《中华人民共和国刑法》，将违反税法构成犯罪的刑罚罚则归并到其第三章第六节，称为（） ["税收犯罪","税务犯罪","危害税收征管罪","涉税犯罪"]
What are three common reasons why customers are saying that they need to evolve their data center? （） ["demand among users for access to information anytime, anywhere","desire to use the same vender for desktop and server deployments","existing equipment has reached and of life","increasing operational costs of managing data centers","architectures that is inflexible and inefficient","selecting best-performing vendors for each individual component"]
合法的行政行为需要满足哪些条件？
