input: Write a python function to convert a dictionary to a JSON string using the `json` module.
  assert dict_to_json({'name': 'John Doe', 'email': 'john@example.com', 'age': 32}) == '{"name": "John Doe", "email": "john@example.com", "age": 32}'
  assert dict_to_json({'fruit': 'Apple', 'size': 'Large', 'color': 'Red'}) == '{"fruit": "Apple", "size": "Large", "color": "Red"}'
  assert dict_to_json({'city': 'New York', 'population': 8000000}) == '{"city": "New York", "population": 8000000}'
output: import json
  
  def dict_to_json(d):
      return json.dumps(d)