input: Write a python function to convert a dictionary to a JSON string using json.dumps.
  assert dict_to_json({"name": "John", "age": 30}) == '{"name": "John", "age": 30}'
  assert dict_to_json({"city": "New York", "is_student": False}) == '{"city": "New York", "is_student": false}'
  assert dict_to_json({"numbers": [1, 2, 3], "status": None}) == '{"numbers": [1, 2, 3], "status": null}'
output: import json
  
  def dict_to_json(d):
      return json.dumps(d)