要使下列程序的输出结果是字符5,则下划线处应填 ( B)#include main(){char letter;int vowel=0;for(letter='A';__;letter++)switch(letter){case 'A':case 'E':case 'I':case 'o':case 'U':vowel++;}printf("%d",vowel);} A. letter<26 B. letter<='Z' C. letter<'Z...