题目内容：

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。

What makes the sea tum red and causes thousands of fish to die？As far back as anyone could remember，the blame was placed on the "red tides. " In 1947，scientists finally connected the red tides with a microscopic sea organism（有机体）1.（call）the dinoflagellate.

The dinoflagellate is so tiny 2. 6，000 of these organisms may be contained in 3. single drop of water. It stands on the borderline between plant and animal in its clasification. it produces its own food，as plants do. But it moves 4.（free）and eats other organisms，as animals do.

Dinoflagellates are normally only one of the many kinds of organisms found in plankton. Plankton is the name given to all very small 5.（form）of sea life. However，when the air and water 6.（be）calm and warm，dinoflagellates multiply with 7.（amaze）sped. The surface of the water appears 8.（cover）with a red carpet.

The dinoflagellates give off a poison. Many fish die. Their bodies are washed up on the beach. Beaches are not fit 9. any use. Fish that are not killed may become 10.（poison）to animals or people who eat them. Commercial fishing comes to a stop.

As dinoflagellates consume all the food and oxygen in an area，they die. After a time，the sea returns to normal. But when conditions are right，the red tide comes again.