Which of the Following Is True of the Pronouns "Who

Which of the following is true of the pronouns "who" and "whom"?

A) Both can introduce restrictive and nonrestrictive clauses.
B) "Who" can only introduce restrictive clauses, while "whom" can only introduce nonrestrictive clauses.
C) "Whom" can only introduce restrictive clauses, while "who" can only introduce nonrestrictive clauses.
D) "Who" can introduce both restrictive and nonrestrictive clauses, but "whom" can only introduce restrictive clauses.
E) "Who" can introduce both restrictive and nonrestrictive clauses, but "whom" can only introduce nonrestrictive clauses.