【单选题】

Imagine fishermen walking down to the seashore, ready to carry out their early morning routine of preparing their boats and net. (1) they hope for a good catch of fish.But to their (2) , a horrible sight meets their still sleepy eyes. Thousands of fish have washed (3) deaD、The cause of this mass destructionA、red tide!

Red tides are a global (4) . They have been observed on both theAtlantic and the Pacific coasts of the United States andCanadA、They have also (5) in many other places. Though relatively few people are (6) them, red tides are not new.
In the Philippines, a red tide was first seen in the province ofBataan in 1908. Since then, red tides have been seen in many other (7) .A、Philippines red tide expert told us that " (8) the fish kills, the Philippines has documented 1, 96 cases of dead shellfish poisoning caused by red tides."
The term “red tide” (9) the discoloration of water that sometimes occurs in certain areas of the ocean or seA、Although the color is often red, it may also be (10) of brown or yellow. The WorldBookEncyclopedia reports that "the discolored areas may range from (11) a few square yards to more than 2,600 square kilometers."
What causes such discoloration Red tides are generally caused by several (12) of single-celled organisms. These tiny organisms have hair-like projections which they use to (13) themselves in water. There are about 2,000 varieties of these organisms, 10 of which carry poisonous (14) These minute organisms usually stay in warm waters with high content of salt.
A、red tide occurs when there is a sudden and rapid (15) of these organisms. The concentration of these organisms may (16) to 50,000,000 per quart of water!Although scientists do not fully understand why this happens, it is known that these organisms (17) when certain conditions simultaneously affect the water. These include abnormal weather, (18) temperatures, an oversupply of nutrients in the water, a generous (19) sunlight, and favorable water currents. When a heavy rainfall occurs, minerals and other nutrients are sometimes washed (20) the land into coastal waters. These nutrients can contribute to the breeding of the organisms. The result Red tides!
A、exceptB、besidesC、despite D、without