21．Correct the errors in the following sentences, if .
A．any B． necessary C． ever D．not
22．—How much do I owe you for lunch?
— .It ' s nothing．
A． You're welcome B． Forget it C． Not at all D．That's right
23．Helping others is a habit, ．you can learn even at an early age
A． it B． which C． what D．that
