why is this code causing runtime error?

Question

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

int main()
{
  char *a = "Hello ";
  const char *b = "World";

  printf("%s", strcat(a, b));
  system("PAUSE");

  return EXIT_SUCCESS;
}

Answer 1

When you are concatenating b to a you are writing into memory you didn't allocate,

Answer 2

Because you are writing data at a memory location that you do not own.

Indeed, when running strcat, you are appending the characters of string b right after the characters of string a. But you haven't claimed for the memory after the string a.