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You are developing a class library. Portions of your code need to access system environment variables.
You need to force a runtime SecurityException only when callers that are higher in the call stack do not have the necessary permissions.
Which call method should you use？（）

You are developing a class library that will open the network socket connections to computers on the network. You will deploy the class library to the global assembly cache and grant it full trust. You write the following code to ensure usage of the socket connections. 
SocketPermission permission =  
new SocketPermission（PermissionState.Unrestricted）； 
permission.Assert（）；
Some of the applications that use the class library might not have the necessary permissions to open the network socket connections.You need to cancel the assertion.
Which code segment should you use？（）