4.18

#include<stdio.h>
main(){
int a,b,c,i=0;
printf("A,B,C三人所选书号分别为:\n");
for(a=1;a<=5;a++)
for(b=1;b<=5;b++)
for(c=1;c<=5;c++)
if(a!=b&&a!=c&&c!=b)
{printf("A:%2d B:%2d C:%2d",a,b,c);
i++;
if(i%4==0)printf("\n");}
printf("共有%d种有效借阅方法\n",i);
}