Which of the following statement is not valid with respect to the use of friend keyword inside a class?

1.10 Which of the following statement is not valid with respect to the use of friend keyword
inside a class?
A) A function may be declared as a friend.
B) A class may be declared as a friend.
C) A private data member can be declared as a friend.
D) A member function of other class can be declared as a friend.