The most ancient vertebrates fossils are of the jawless fish. The oldest rocks in which these fossils are found are strata of the Ordovician period, just above the Cambrian. It must, therefore, be true that:

A) jellyfish are not present in Precambrian rocks.
B) animals with gill slits lived in the Ordovician.
C) graptolites evolved into jawless fish.
D) vertebrates with true jaws first appeared in the Jurassic, just above the Ordovician.