单项选择题

Ash from burning coalfields, sparked by massive volcanic eruptions (爆发), may have added to the crises that led to the biggest extinction (绝迹) event in Earth’s history.
Many factors (67) to cause the mass extinction 251 million years ago, during which about 96 percent of (68) species and 70 percent of those on land went extinct. Several researchers have speculated that volcanism might have set (69) extensive coal fires that contributed to the extinction, but no clear (70) had been found—until now.
In the stuff from the Sverdrup basin in the Canadian Arctic, a team (71) by Stephen Grasby at the Geological Survey of Canada in Calgary found tiny spherical (球形颗粒) (72) that are evidence of open coal burning—very different from the ash that comes from (73) vegetation (植物). Stephen Grasby speculates that major volcanic eruptions (74) 1000 kilometres east of the Sverdrup basin—in (75) is now Siberia—may have lighted overlying coal (76) and released huge quantities of ash (77) the atmosphere.
Modern-day coal-fired power plants remove this so-called "fly ash" from their deposits because it (78) a lot of toxic metals. When Grasby looked more closely at the stuff (79) to the early period of history, he found that they, too, contained high (80) of toxic metals. This suggests that fly ash may have (81) ancient oceans and lakes and contributed to the extinction.
Further studies of other prehistoric sediments around the world should show how (82) the fly ash spread as the coalfields burned, and therefore how important it is likely to have been in causing the (83) extinction, says Andrew Knoll, a professor at Harvard University. If it (84) to be one of the main (85) of the extinction, he notes, it would probably have killed off species (86) indiscriminately. Another factor must explain why some groups suffered massive extinctions, while others did not, he says.