input: Write a python function to convert a dictionary to a JSON string.
  assert dict_to_json({"name": "John", "age": 30, "car": ["Ford", "BMW", "Fiat"]}) == '{"name": "John", "age": 30, "car": ["Ford", "BMW", "Fiat"]}'
  assert dict_to_json({"city": "New York", "population": 8000000}) == '{"city": "New York", "population": 8000000}'
  assert dict_to_json({"fruit": "Apple", "quantity": 5, "in_stock": True}) == '{"fruit": "Apple", "quantity": 5, "in_stock": true}'
output: import json
  
  def dict_to_json(d):
      return json.dumps(d)