At the End of the Permian Era 250 Million Years

At the end of the Permian era 250 million years ago,70% of all species living on land and 90% of all species living in the oceans disappeared.What is believed to have been the cause of this massive extinction?

A) global warming from volcanic gases released during the breakup of the Gondwanaland supercontinent
B) a nearby supernova (the explosion and total destruction of a star)
C) devastating impact by an asteroid
D) a "snowball Earth" episode in which global cooling caused the EarthEarth''s ocean surfaces to freeze and glaciers to cover the continents