Help me in solving GSCP14 problem

My issue

My code

// Update the '_' in the code below

#include <stdio.h>
#include <string.h>

#define MAX_LEN 1001 // maximum length of input string

int main() {
    int t;
    scanf("%d", &t);
    char S[MAX_LEN]; // declare a character array to store the input string
    for (int i = 0; i < t; i++) {
        scanf("%s", S); // read in the input string
        int len = strlen(S); // get the length of the input string
        char X[MAX_LEN * 2]; // declare a character array to store the concatenated string
        int j;
        for (j = 0; j < len; j++) {
            X[_] = S[_]; // copy S into X
        int k;
        for (k = 0; k < len; k++) {
            X[_+_] = S[_]; // concatenate S with itself
        X[j+k] = '\0'; // terminate the string
        printf("%s\n", X); // output the concatenated string
    return 0;

Learning course: C for problem solving - 1
Problem Link: CodeChef: Practical coding for everyone

In this code u have to replace _ with some logical characters so that the codes in syntactically and logically correct . U code should output the given string two time which means u have to concatenate them and store it in another string of twice the size of the given string.