input: Write a function to classify programming languages based on their level.
  assert classify_languages("Python") == "High-level programming language"
  assert classify_languages("Java") == "High-level programming language"
  assert classify_languages("C") == "Low-level language"
  assert classify_languages("C++") == "High-level language"
  assert classify_languages("JavaScript") == "High-level language"
  assert classify_languages("Ruby") == "Unknown language"
output: def classify_languages(language):
      high_level = {"Python", "Java", "C++", "JavaScript"}
      low_level = {"C"}
  
      if language in high_level:
          return "High-level programming language"
      elif language in low_level:
          return "Low-level language"
      else:
          return "Unknown language"