input: Write a python function to resolve a domain name to its corresponding IP address. 
  assert resolve_domain('google.com') is not None
  assert resolve_domain('thisdomaindoesnotexist.tld') is None
  assert isinstance(resolve_domain('example.com'), str)
output: import socket
  
  def resolve_domain(domain):
      try:
          return socket.gethostbyname(domain)
      except socket.gaierror:
          return None