input: Write a function to parse a JSON data structure and extract values for specified keys. If the keys are not present, the function should return None.
  
  
  assert parse_json('{"name": "John", "age": 18, "occupation": "Software Engineer"}')==("John", 18, "Software Engineer")
  assert parse_json('{"name": "Alice", "age": 22, "occupation": "Data Scientist"}')==("Alice", 22, "Data Scientist")
  assert parse_json('{"name": "Bob", "occupation": "Teacher"}')==("Bob", None, "Teacher")
  assert parse_json('{"age": 25}')==(None, 25, None)
  assert parse_json('{}')==(None, None, None)
  
output: def parse_json(json_str):
      data = json.loads(json_str)
      name = data.get("name")
      age = data.get("age")
      occupation = data.get("occupation")
      return (name, age, occupation)