题目内容：
A blue hole is a flooded sea cave with a hole that opens up at the land's surface. Some blue holes have very special rock___26___ and water chemistry. Far below sea level, they contain some of the___27___ environments on earth, with no oxygen and no light. Yet these areas are filled with life forms that have___28___ the extreme conditions. Information gathered from these blue holes is helping scientists increase their understanding of biology, archaeology (考古学) and___29___. Blue holes get their name from the color some have when seen from the air. The color is usually a___30___ the sky on the water. But not all of these cave systems have blue surfaces. Some contain dark or___31___ water.One reason blue holes have not been fully explored is that they can be extremely dangerous. There are many safety rules that divers must follow to help ensure their ___32___. Scientists are interested in these caves because oxygen-free conditions there are___33___ those on earth long ago, before oxygen existed on our planet. Experts are not only interested in life on our planet. They study these extreme conditions to understand how and where life might exist on other planets, Blue hole's also permit scientists to study climate change over thousands of years. They want to understand what those changes could mean in the future. Because blue holes contain no oxygen, they also protect ancient objects fromthe___34___of time.
However, many blue holes have become areas where people throw away waste. These actions are polluting an important source of fresh water. One of the researchers' goals is to raise___35___ about these underground areas solocal communities will take better care in protecting them.
第26题_______
参考答案：
答案解析：