Elevated Levels of Iridium Are Found in the Geologic Layer

Elevated levels of iridium are found in the geologic layer at the K-T boundary.Is this unusual?

A) Yes. Iridium is a common metal on Earth but rarely accumulates in a singe layer.
B) No. Iridium is a rare element on Earth, but geological layers of iridium are not uncommon.
C) Yes. Iridium is a rare metal on Earth.
D) Yes. Composition differences between geological layers tend to equilibrate over time.