Iridium is a substance found only in asteroids and other space objects. This graph shows the amount of Iridium found in different sediment layers of Earth. They are marked off into 17 sections representing time.

Iridium Levels graph shows Iridium levels in sediment layers. Beginning at sediment layer 1, iridium levels remain between 0 and 5, jumping to a level of 23 between layers 5 and 6, and decreasing to near starting levels at layers 14 through 17.

What most likely happened in sediment layer 5?

A massive volcano erupted, causing global warming and lava flow, which in turn caused mass extinction.
More iridium was produced by space objects and dumped by local factories.
A massive dust cloud formed, causing global cooling and a lack of sunshine. This in turn caused a lack of plants and a mass extinction.
Fewer impacts lead to a drop in iridium levels and a mass extinction caused by a lack of sunshine.