21.雲端運算可以分為幾個層次，下列何者配對不正確？ (A) Google Docs /軟體即服務 (Software as a service) (B) Microsoft Azure /基礎設備即服務(Infrastructure as a service) (C) Amazon EC2/基礎設備即服務 (Infrastructure as a service) (D)Google App Engine /平台即服務 (Platform as a service)。