为什么用scanf_s和gets都会显示错误（用的VS2017）？

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

#define MAX_STAGE 10  /*关卡数*/
#define LEVEL_MIN  3  /*最低等级*/
#define LEVEL_MAX 20  /*最高等级*/

int sleep(unsigned long x)
{
    clock_t c1 = clock(), c2;

    do {
        if ((c2 = clock()) == (clock_t)-1)  /*错误*/
            return 0;
    } while (1000.0 * (c2 - c1) / CLOCKS_PER_SEC < x);
    return 1;
}

int main()
{
    int i, stage;
    int level;
    int success = 0;
    clock_t start, end;
    const char ltr[] = "ABCDEFGHIJKLMNOPQSYUVWXYZ";

    srand(time(NULL));

    printf("英文字母记忆训练\n");

    do {
        printf("输入要挑战的等级（%d~%d）", LEVEL_MIN, LEVEL_MAX);
        scanf_s("%d", &level);
    } while (level < LEVEL_MIN || level > LEVEL_MAX);

    printf("来记忆一个%d个英文字符吧。\n", level);

    start = clock();
    for (stage = 0; stage < MAX_STAGE; stage++)
    {
        char mstr[LEVEL_MAX + 1];
        char x[LEVEL_MAX * 2];

        for (i = 0; i < level; i++)            /*生成作为题目的字符串*/
            mstr[i] = ltr[rand() % strlen(ltr)];
        mstr[level] = '\0';

        printf("%s", mstr);
        fflush(stdout);
        sleep(1000 * level);          /*问题提示时间*/

        printf("\r%*s\r请输入： ", level, "");
        fflush(stdout);
        //scanf_s("%s", x);  /*此处应该怎么改*/
        gets(x);

        if (strcmp(x, mstr) != 0)
            printf("\a回答错误。\n");
        else
        {
            printf("回答正确。\n");
            success++;
        }
    }
    end = clock();

    printf("%d次答对了%d次。\n", MAX_STAGE, success);
    printf("用时%.1f秒。\n", (double)(end - start) / CLOCKS_PER_SEC);

    system("pause");
    return 0;
}

scanf_s("%s", x, LEVEL_MAX * 2 - 1);