input: Write a python function to extract keys from a dictionary.
  assert extract_keys({'name': 'John', 'age': 24, 'gender': 'male'})==['name', 'age', 'gender']
  assert extract_keys({'city': 'New York', 'country': 'USA', 'population': 8000000})==['city', 'country', 'population']
  assert extract_keys({})==[]
output: def extract_keys(d):
      return list(d.keys())