input: Write a python function to concatenate two strings and print the result.
  assert concatenate_strings("Hello", "World") == "HelloWorld"
  assert concatenate_strings("Python", "Programming") == "PythonProgramming"
  assert concatenate_strings("Data", "Science") == "DataScience"
output: def concatenate_strings(str1, str2):
      return str1 + str2