In the late 1700s scientists proposed that the layers in the rocks of mountains and canyons represented a record of the passage of time, with the younger layers lying on top of the older layers. what could they determine about the fossils they were finding in the rocks based on this observation?
a. fossils in upper layers were younger than fossils in lower layers.
b. fossils in upper layers were less weathered and more detailed than those in lower layers.
c. the exact age of a fossil could be determined by counting the rock layers.
d. the oldest fossils were the same as the ones found in younger rock layers.

The correct answer is - A. Fossils in upper layers were younger than fossils in lower layers.

The scientists managed to determine that the rock layers of the mountains and the canyons are arranged by time, and that they are a nice representation of the geological past. Because the scientists proposed this, that also means that the rock layers are undisturbed by geological process, thus they are arranged by age, one over the other, with the oldest being at the bottom, and the youngest at the top.

When it comes to the fossils, this meant that now the scientists can now the approximate age of the fossils, depending on the layer in which the fossils are found. So the closer they are found to the surface, the younger they are, and the deeper they found, the older they, the same as the rock layers.