input: from typing import List
  
  
  def generate_extension(class_name: str, extensions: List[str]) -> str:
      """Given the name of a class (a string) and a list of extension names, generate a class name with the extension.
      The class name is the concatenation of the class name and the first extension's name.
      
      >>> generate_extension('my_class', ['extension1', 'extension2'])
      'my_class.extension1'
      >>> generate_extension('base', ['utility'])
      'base.utility'
      """
      
      return class_name + '.' + extensions[0]
output:     return class_name + '.' + extensions[0]