An Iridium-Rich Layer of Clay,glass Spherules,and Shocked Quartz Grains Strongly

Multiple Choice

An iridium-rich layer of clay,glass spherules,and shocked quartz grains strongly suggests that the K-T boundary extinctions were at least in part caused by

A) a long trend of global warming.
B) a long trend of global cooling.
C) worldwide explosive volcanism.
D) the impact of a bolide (meteoroid or comet) .