2.) How is the layer of iridium used to tell the relative age of different rock strata and fossils?

The layer of iridium is used to tell the relative age of different because the iridum was deposited worldwide and the comparison can be made world wide.

chordata is the answer

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.