choose the correct answer to fill in the blanks.
avalanche

on march 21, 2002, steven borg, a swiss ski enthusiast has been skiing/skied/has skied/skies/was skiing,
near his hometown, when something unexpected has happened/happens/is happening/was happening/happened : "there was a terrible avalanche. suddenly i was flying/am flying/have flown/flew/have been flying through the air with my hands above my head and a sea of white all around then, i have found/was finding/found/find/am finding myself one foot deep under the snow. the snow was feeling/has felt/felt/is feeling/feels like cement and i am fighting/was fighting/have fought/fight/have been fighting to free myself when i heard/can hear/have heard/hear/was hearing some people trying to rescue me."
this is how he explained the disaster to his rescuers afterwards. borg was lucky that he has survived/is surviving/survives/survived/was surviving the disaster, but not everybody is so lucky.
every year, avalanches - or snowslides - kill/have killed/killed/were killing/have been killing about 200 people worldwide, and deaths are not the only part of the problem. avalanches also caused/have caused/have been causing/cause/were causing
billions of dollars of damage annually to buildings, roads and livestock, and have threatened/were threatening/threatened/have been threatening/threaten many mountain communities.
in switzerland, for example, an estimated 65% of the population lives/was living/has lived/lived/has been living in areas at risk for avalanches. about two decades ago, most countries are beginning/began/were beginning/begin/have begun
establishing networks of centers to monitor and forecast snowslides, an action which was helping/has helped/helped/is helping/helps
to limit the number of victims since. it so works that the centers warn/have warned/are warning/were warning/warned people when a high risk of avalanche has existed/existed/is existing/exists/was existing and are advising/advised/advise/have advised/were advising skiers to avoid dangerous areas, and in the meantime, authorities have started/started/were starting/are starting/start
taking the necessary steps to protect people and infrastructure. so far, such passive defenses against avalanches are including/include/included/have included/were including
evacuating villages, bridges, roads and ski resorts. and over the past years, scientists have been exploring/are exploring/explore/were exploring/explored
the possibilities of active prevention of avalanches