题目要求：找出所有的四位数——它的千位和十位组成的新十位数是素数，百位和个位组成的十位数也是素数，按大小顺序输出这些数，并计算其个数。程序如下：
using System;
class prime
{
    static void Main()
    {
        int x,y,i,j,p,q,c,d,z;
        int n=1;
        
        for(i=1;i<=9;i++)
            for(j=1;j<=9;j++)
            {
                p=i*10+j;
                for(c=2;c<=Math.Sqrt (p);c++)
                    if(p%c==0)
                        break;
                    else
                        for(x=1;x<=9;x++)
                            for(y=1;y<=9;y++)
                            {
                                q=x*10+y;
                                for(d=2;d<=Math.Sqrt (q);d++)
                                    if(q%d==0)
                                        break;
                                    else
                                        z=i*1000+x*100+j*10+y;
                                        //int[n] Number= new z;
                                //Console.Write ("{0}",Number[n]);
                                n++;
                                Console.Write ("{0}",n);
                            }
            }
    }
}

倒数七八行没有“//”符号。