c语言，关于pointer的问题，求解，THX

原程序修改如下：
//---------------------------------------------------------------------------
#include <stdio.h>
int main(int argc,char **argv){
char hello[]={'h','e','l','l','o','\0'};
char *s;
char *hello2 = "hello";/*注意这里*/
char *c,*d;/*注意这里，C语言中，变量的定义一定要放在执行语句之前。*/
int len;

s = hello; // s now points to hello[0]
// could be "s = &hello[0];"
len = 0;
while(s[len] != '\0'){
len++;
}
hello[0]='y';

hello[0]='y'; //impossible

printf("len = %d\n",len);
printf("hello2[0] = %c\n",hello2[0]);
printf("hello[0] = %c\n", hello[0]);
printf("*hello = %c\n", *hello);
printf("s = %c\n",s[3]);
printf("s - hello = %d\n", s - hello);

c = &s[0];/*注意这里*/