The retreat of the shallow seas of the paleozoic left much of virginia exposed during the permian and mesozoic. in central virginia, trace fossils, including footprints have been found

The given blank can be filled with dinosaur footprints.  

In the Precambrian, the sedimentary rocks deposition took place in the Blue Ridge Province of Virginia, however, no fossils have been positively determined from these deposits. The Paleozoic rocks are well documented in Virginia. The occurrences of shallow area deposition were obstructed by subsequent periods of erosion and mountain-building incidents.  

The withdrawal of the widespread shallow regions of the Paleozoic left much of the state exposed at the time of Mesozoic and Permian. In central Virginia, fossils of freshwater fish, dinosaur footprints, and insects are found in the rift basin deposits of the Triassic.  

the answer is b.) hydrogen bonds between bases.