简述对虾软壳病的病因、症状及防治方法。
参考答案：
（1）病因：
A.饵料不足或营养不全（Ca和P含量过少或含量不均衡）
B.饲料储存不当，腐败变质
C.水体水质变差，有毒物质积累
D.水体pH过高，导致P以CaPO4沉淀，使水体可溶性P减少
E.水体中含有有机锡或有机磷等消毒剂
（2）症状：甲壳薄而软，壳与肌肉分离
（3）防治：
A.改善水质，加大换水量；
B.多投放鲜饵，如投放贝肉等；
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| The first day of school our teacher let us know someone. Looking out, we found an old woman 1 at the door. She said,"Hi, my name is Susan. I am seventy years old". While we were 2 what made her take this 3 at her age. She said with a smile."I'm here to meet a rich husband, get 4 , have two children and travel together." Hearing this, all of us laughed."In fact,"she went on," I always 5 of having a college education, and now I'm getting 6 !" After class we had chocolate and milk together. We became good 7 . I often listened to this "time machine" as she shared (分享) her 8 with me. At the end of the term we invited Susan to give us a 9 . When she was stepping up to the podium (讲台), suddenly she found she took the wrong paper. She felt a little 10 and said, "I am sorry. I am so nervous. I can't get my ready talk 11 , so I have to tell you 12 I know." In her speech she told us" We don't stop playing as we are old, we grow old 13 we stop playing. The secret of staying 14 , being happy and getting success is that we should laugh and find something 15 every day. I think there is a great 16 between growing old and growing up. I am seventy now. If I 17 in bed for one full year and never do anything, I'll become seventy-one. Since anybody can grow older, my 18 is to grow, up by always finding the chance in change. Growing older can't be avoided (避免), growing up can be 19 . It's never too late to be all you can possibly be." During the four years' study, it was so easy for Susan to make friends 20 she went. In the end, the wonderful woman got the college degree (学位). | ||||
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