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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_default_id(credentials=None):
 """ Get default project id. Returns: the default project id if there is one, or None. """

    project_id = _utils.get_project_id()
    if project_id is None:
      projects, _ = Projects(credentials)._retrieve_projects(None, 2)
      if len(projects) == 1:
        project_id = projects[0].id
    return project_id