听力原文：As part of our general IT upgrade, we've been seriously overhauling our security

infrastructure — partitioning information across multiple servers, using new encryption schemes, and so on. So one of the ways this is going to impact all of you is — well, aside from making your data more secure, we're adopting a new password policy, and we're asking everyone to please understand it and comply with it. Basically, what the policy means is... every password you use within the corporate network —whether it's for your employment records or project files — all passwords now have a lifespan of one year. That means that once a year, you're going to have to change all your passwords. I know it's a small inconvenience to you, but it adds a very important layer of network security... and I hope you'll all agree with how important THAT is.

Why has the speaker given this talk？

A．To alert staff to a problem with network security

B．To announce a new policy related to network security

C．To recommend a method for improving network security

D．To inform. new employees about the company's network security policies