input: Write a python function to retrieve keys from a dictionary.
  assert retrieve_keys({"name":"John", "age":30, "occupation":"teacher"})==["name", "age", "occupation"]
  assert retrieve_keys({"brand":"Toyota", "model":"Corolla", "year":2020})==["brand", "model", "year"]
  assert retrieve_keys({"country":"Japan", "population":126476461, "continent":"Asia"})==["country", "population", "continent"]
output: def retrieve_keys(d):
      return list(d.keys())