which of the following pieces of evidence prompted researchers to abandon the hypothesis that a supernova caused the high levels of iridium in the k-t layer? select one response group of answer choices coarse sediment mixed with uniform cretaceous mud in texas differences in foraminifera fossils above and below the k-t boundary layer high levels of iridium in the k-t boundary layer lack of an isotope of plutonium in the k-t boundary layer