input: Write a python function to extract the keys from a dictionary.
  assert extract_keys({'name': 'John', 'age': 25, 'address': 'London'})==['name', 'age', 'address']
  assert extract_keys({'name': 'Alice', 'age': 30, 'city': 'New York'})==['name', 'age', 'city']
  assert extract_keys({})==[]
output: def extract_keys(d):
      return list(d.keys())