Kerberos是一种使用对称密钥加密算法来实现通过可信第三方密钥分发中心的身份认证系统。()

A. could be doing more to protect the environment
B. cares deeply about protecting the environment
C. refuses to pay attention to the issues facing the environment
D. is actively seeking to damage the environment

A. Because they are a part of the environment.
Because of twenty-five years of progressive environmental legislation.
C. Because they set the standard for land stewardship in America.
D. Because millions of people visit them each year.