input: Write a python function to classify programming language based on keywords in the code snippet.
  assert classify_programming_language('def my_function():\n    print("Hello World!")') == 'python'
  assert classify_programming_language('public static void main(String[] args) {\n    System.out.println("Hello World!");\n}') == 'java'
  assert classify_programming_language('var greeting = "Hello World!";\nfunction sayHello() {\n    console.log(greeting);\n}') == 'javascript'
  assert classify_programming_language('int main() {\n    printf("Hello World!");\n    return 0;\n}') == 'c'
  assert classify_programming_language('print("This is a test")') == 'python'
  assert classify_programming_language('System.out.println("Another test")') == 'java'
output: def classify_programming_language(code_snippet):
      if 'def ' in code_snippet or 'print(' in code_snippet:
          return 'python'
      elif 'public static void main' in code_snippet or 'System.out.println' in code_snippet:
          return 'java'
      elif 'var ' in code_snippet or 'console.log' in code_snippet:
          return 'javascript'
      elif 'int main()' in code_snippet or 'printf(' in code_snippet:
          return 'c'
      else:
          return 'unknown'