A geologist studies a layered rock to determine its age. Bacterial fossils from Precambrian time are found in the rock’s upper layers, but no bacterial fossils are found in the rock’s bottom layers. Which of these conclusions about the fossils within the rock is best supported by the evidence?

A. The age of the entire rock is determined by the age of the bacterial fossils within it.
B. Most Precambrian organisms were too small to leave fossil remains so the age of the rock cannot be determined by examining this evidence.
C. Because bacterial fossils are found only in the upper layers, scientists can conclude that these bacteria evolved towards the end of Precambrian time.
D. The bacterial fossils must have been formed during a different time period because most of the rocks from the Precambrian have been destroyed due to tectonic activity.