distractors: where what when
sentence: It is well known of the   dangers   of a possible earthquake, and for centuries man has been making researches on earthquakes.More than 2,000 years ago,   for example  , a Chinese scientist named Zhang Heng   invented   a machine which could find out from which   direction   the seismic waves had come, and this machine is still   used   by scientists today.Now we know much more about earthquakes and   [MASK]   they happen, but we still cannot   tell   exactly when and where an earthquake will happen, and cannot   stop   it from happening.So earthquakes are among the   worst   disasters in the world.