微生物有着广泛的用途，下列对有关微生物应用的叙述中错误的一项是（　　）

阅读下列材料：

材料一：

材料二：  朝廷在故都（东京）时，实仰东南财赋，而吴（江苏）中又为东南根抵。语曰“苏常熟，天下足”。

请回答：

(Some classmates are talking about their weekend.)

    Linda: I stayed at home and studied over the weekend. I studied for my English test on Saturday. Yesterday I wrote a geography paper. In the evening I talked to my pen pal (笔友) on the phone. What did you do?

Tim: I had a good time. On Saturday morning I played tennis, in the afternoon I went to the beach, and in the evening I went to Nancy's birthday party. On Sunday I went to the mountains with my friends.

Mary: Oh, your weekend was great. But mine was boring. I stayed at home the whole (全)weekend. I cleaned my room and washed my clothes. I even watered the flowers. And only in the evening I watched TV for a short time. Did you go out, Dave?

Dave: No, I didn't. I stayed at home and watched TV on Saturday. Yesterday I watched a movie on TV, Fearless. It was very exciting.

养鱼缸里经常要放一些新鲜的水草，这样做的主要目的是（　　）