指针与数组0xc0000005 Access Violation问题

这是谭浩强书中的代码，请大家帮忙找下原因：

==============这是使用数组表示法正常运行的版本===================
#include<stdio.h>

copy_string(char from[],char to[]){
int i=0;
while(from[i]!='\0'){
to[i] = from[i];
i++;
}
to[i] = '\0';
}

main(){
char a[] = "I am a teacher.";
char b[] = "you are a student.";
printf("string a is:%s\nstring b is:%s\n",a,b);
copy_string(a,b);
printf("string a is:%s\nstring b is:%s\n",a,b);
}

=================================================================

==================这是使用指针表示法报错的版本===================
#include<stdio.h>

copy_string(char from[],char to[]){
int i=0;
while(from[i]!='\0'){
to[i] = from[i];
i++;
}
to[i] = '\0';
}

main(){
//char a[] = "I am a teacher.";
//char b[] = "you are a student.";
char *a = "I am a teacher.";
char *b = "you are a student.";

printf("string a is:%s\nstring b is:%s\n",a,b);
copy_string(a,b);
printf("string a is:%s\nstring b is:%s\n",a,b);
}

=============================================================

恳请各位朋友帮忙解释下为何报如下错误：
0xc0000005 Access Violation

谢谢大家！

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1.ch