21. As ________ rule, apples are sold by ______ weight and eggs by _______dozen.
A. a; 不填；the B. a; the; the C. a; a; the D. the; 不填；不填
22.The time he has devoted in the past years _______ the disabled is now considered ________ of great value.
A. to help; being B. to helping; to be C. to help; to be D. to helping; being
23. ___________both sides accept the agreement will a lasting peace be established in this region.
A. If only B. Only if C. unless D. As long as
24. Every boy and every girl as well as some teachers who_____ to visit the museum______ asked to be at the school gate before 6 :00 in the morning.
A. are ; are B. is; is C. is ;are D. are ;is
