1、

#include <stdio.h>

int main(void)
{
    int i;
    puts("please input an integer.");
    printf("i = "); scanf("%d", &i);
    
    if (i % 3 == 0)
        puts("can be devided");
    else
    {
        if (i % 3 == 1)
            puts("remainder is 1.");
        else
            puts(
 
"remainder is 2.");
    }
    return 0;
}

↓

#include <stdio.h>

int main(void)
{
    int i;
    puts("please input an integer.");
    printf("i = "); scanf("%d", &i);
    
    switch (i % 3)
    {
        case 0 : puts("can be devided."); break;
        case 1 : puts("remainder is 1."); break
 
;
        case 2 : puts("remainder is 2."); break;
    }
    return 0;
}

2、

#include <stdio.h>

int main(void)
{
    int i;
    puts("please input an integer.");
    printf("i = "); scanf("%d", &i);
    
    switch (i)
    {
        case 1 : puts("a"); puts("b"); break;
        case 2 : puts("
 
c");
        case 3 : puts("d"); break;
        case 4 : ; break;
        case 5 : puts("e"); break;
        case 6 : 
        case 7 : puts("f"); break;
        default : puts("g"); break;
    }
    return 0;
}