[c-prog] please help

[titli_juit]

     #include<stdio.h>
int main()
        {
             int i=0;
         for(i=0;i<20;i++)
            {
                         switch(i){
               case 0:i+=5;
               case 1:i+=2;
               case 5:i+=5;
               default : i+=4;
                                   break;}
                 
               printf("%d,",i);
               }    
         
return 0;
}

the output is 16,21

can you please explain how am i getting this output?