What is the difference between the inner and outer identity?

What is the difference between the inner and outer identity?
A . Only the authentication server provides its credentials in the outer identity response.
B . The inner identity is only for authentication server credentials provided to the supplicant.
C . The inner identity must correspond to the outer identity for realm-based authentications.
D . The outer identity is in plain text; the inner identity is securely transmitted inside a TLS tunnel.
E . The outer identity is only for authentication server credentials provided to the supplicant.

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Answer: D