input: Write a python function to convert a dictionary to a JSON string using the json module.
  assert dict_to_json({"name":"John Doe","age":18,"hobbies":["reading","swimming","gaming"],"contact":{"email":"johndoe@example.com","mobile":"+1900000000"}}) == '{"name":"John Doe","age":18,"hobbies":["reading","swimming","gaming"],"contact":{"email":"johndoe@example.com","mobile":"+1900000000"}}'
  assert dict_to_json({"city":"New York","population":8500000,"landmarks":["Statue of Liberty","Central Park"]}) == '{"city":"New York","population":8500000,"landmarks":["Statue of Liberty","Central Park"]}'
  assert dict_to_json({"company":"OpenAI","employees":1500,"products":["GPT-3","DALL-E"]}) == '{"company":"OpenAI","employees":1500,"products":["GPT-3","DALL-E"]}'
output: import json
  
  def dict_to_json(d):
      return json.dumps(d)