47 4.3x+11y=55,已知x,y为正整数,则x+y=( ) A.

10 B.11 C.12 D.13 5.5x+6y=76,且x,y均为质数,则x+y=( ) A.11 B.12 C.13 D.14 6.2x+7y=21,已知x,y为正整数,则x-y=( ) A.4 B.5 C.6 D.7