Complete the sentences with a word derived from the given words

For decades, [ ] flashesof light have been seen above the [ ] valley of Hessdalen in Norway. This [ ] phenomenon began in1981, when hundreds of sightings were reported by [ ] eye-witnesses. Scientists wereso intrigued by the phenomenon that they installedadvanced computerised [ ] tomonitor the area. But despite 25 years of research,the mystery continues to baffle the [ ] community. One theory is that the lightsare caused by water seeping into rocks in the valley.When this water freezes, it is thought to generate [ ]. This spark could thenignite gases in the air, creating a flash of light.However, there is little [ ] tosupport this theory and it has been heavily criticisedas being physically [ ].Sceptics have suggested that the lights could becaused by planes approaching a nearby airport, butas the lights have been seen in the sky for hours, thistheory seems quite [ ]. Whateverthe cause of the mystery, it is a [ ] sight to see and attracts countless [ ] to the area each year.