题目

 The 2016 Summer Olympic Games will be held in Rio de Janeiro, Brazil, which becomes the first city in South America _________Olympics.      A. hosting                      B. to host     C. having hosted                    D. being hosted 答案：B13．下列各组细胞器均无膜结构的是（　　）A．液泡和叶绿体B．中心体和核糖体C．内质网和线粒体D．溶酶体和高尔基体