A. HOW B. WHAT C. THAT D. WHICHREAD THE FOLLOWING PASSAGE AND MARK...

31. A. how B. what C. that D. whichRead the following passage and mark the letter A, B, C, or D on your answer sheet toindicate the correct answer to each of the questions.It’s a sound you will probably never hear, a sickened tree sending out a distress signal. But agroup of scientists has heard the cries, and they think some insects also hear the trees and aredrawn to them like vulture to a dying animal. Researchers with the US Department ofAgriculture’s Forest Service fastened sensors to the bark of drought-stricken trees clearlyheard distress calls. According to one of the scientists, most parched trees transmit theirplightin the 50-hertz to 50-kilohertz range. (The unaided human ear can detect no more than20 kilohertz). Red oak, maple, white pine, and birch all make slightly different sounds in theform of vibrations at the surface of the wood.The scientists think that the vibrations are created when the water columns inside tubes thatrun along the length of the tree break, a result of too little water following through them.These fractured columns send out distinctive vibration patterns. Because some insectscommunicate at ultrasonic frequencies, they may pick up the trees' vibration and attack theweakened trees. Researchers are now running tests with potted trees that have been deprivedof water to see if the sound is what attracts the insects. “Water-stressed trees also smelldifferently from other trees, and they experience thermal changes, so insects could beresponding to something other than sound”, one scientist said.