input: Write a function to concatenate two strings "Hello" and "World" and print them in the same line with a space between them.
  assert print_strings()=="Hello World"
  assert print_strings()=="Hello World"
  assert print_strings()=="Hello World"
  assert print_strings()=="Hello World"
  assert print_strings()=="Hello World"
output: def print_strings():
      return "Hello World"