题目

阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。Across the globe today, there 【1】 (be) still so many challenges to be solved when it comes to food security. With the Earth’s population expected to three times in 【2】 (little) than a century, the ongoing struggle to feed the present population continues to be a difficult problem across the globe, there is an 【3】 ( increase) anxiety for the agricultural department. Not only do we need 【4】 (improve) the practical usage of food across the globe, but also we should broaden the crop stra若某动物细胞内有两对同源染色体，分别用A和a，B和b表示。下列各组精子中由同一个精原细胞经减数分裂形成的是A．AB、ab、ab、AB B．AB、aB、aB、ABC．AB、Ab、aB、ab D．aB、aB、ab、ab