Rightmost derivation:

<assign> => <id> = <expr>
                   => <id> = <expr> + <term>
                   => <id> = <expr> + <term> * <factor>
                   => <id> = <expr> + <term> * <id>
                   => <id> = <expr> + <term> * A
                   => <id> = <expr> + <factor> * A
                   => <id> = <expr> + <id> * A
                   => <id> = <expr> + C * A
                   => <id> = <term> + C * A
                   => <id> = <factor> + C * A
                   => <id> = <id> + C * A
                   => <id> = B + C * A
                   => A = B + C * A