如何修复"变量先前声明'静态'重新声明'extern'错误"？

Question

当我尝试:

#include <stdio.h>

static int c1 = 10;

int main(){

    {
        extern int c1; //c1 here still has internal linkage
        printf("%d",c1);
    }

return 0;
}

什么都不会出错.但是当我尝试修改它时有点像这样:

#include <stdio.h>

static int c1 = 10;

int main(){

    int c1 = 20; //This is where I modified.
    c1++;

    {
        extern int c1;
        printf("%d",c1);
    }

return 0;
}

突然间,出现了一个错误.它说"变量先前声明'静态'重新声明'extern'".为什么？

逐字错误消息:

prog.c: In function ‘main’:
prog.c:11:20: error: variable previously declared ‘static’ redeclared ‘extern’
         extern int c1;