补全空缺短句，奖励100HY^^[cloze] In Southern China, caves aren’t just used for shelter; they can be__1__. People have been visiting this cave for generations. The cave floor is covered in guano, so plentiful that ten minutes’ work can fill this farmer’s baskets. It’s used as__2__.

A clue to the source of the guano can be heard above the noise of the river. The sound originates high up in the roof of the cave. The entrance is full of swifts. They’re very__3__. More than 200,000 of them share this cave in southern Guizhou Province, __4__in China. These days, Chinese house swifts mostly nest in the roofs of buildings. But rock crevasses like these were their original home, long before houses were invented.

Though the swifts depend on the cave for shelter, __5__, as their eyes can’t see in dark. However, deep inside the cavern, other creatures are better equipped for subterranean life. A colony of bats is just waking up, using ultrasonic squeaks to orientate themselves in the darkness. Night is the time to go hunting.

Rickett's mouse-eared bat is the only bat in Asia which__6__, tracking them down from the sound reflection of ripples onto water surface. This extraordinary behavior was only discovered in the last couple of years and has never been filmed before.If catching fish in the dark is impressive, imagine eating a slippery minnow__7__.[/cloze]