—       Thank you for inviting me.

算一算。

$\frac{\text{4}}{\text{7}}\text{+}\frac{\text{2}}{\text{7}}\text{=}$ ____          $\frac{\text{1}}{\text{2}}\text{+}\frac{\text{1}}{\text{2}}\text{=}$ ____         $\frac{\text{6}}{\text{9}}\text{-}\frac{\text{5}}{\text{9}}\text{=}$ ____         $\frac{\text{3}}{\text{4}}\text{+}\frac{\text{1}}{\text{4}}\text{=}$ ____

$\frac{\text{8}}{\text{9}}\text{-}\frac{\text{4}}{\text{9}}\text{=}$ ____           $\frac{\text{1}}{\text{5}}\text{+}\frac{\text{2}}{\text{5}}\text{=}$ ____        $\frac{5}{7}-\frac{5}{7}=$ ____         $1-\frac{7}{10}=$ ____

①加抗生素  

②不加抗生素  

③加氮素  

④不加氮素  

⑤加葡萄糖

⑥不加葡萄糖 

⑦37℃恒温箱培养  

⑧28℃﹣﹣30℃温度下培养．