Why Did Evolutionary Biologists Rely Exclusively on the Concept of Geologic

Why did evolutionary biologists rely exclusively on the concept of geologic time to explain macroevolutionary change for such a long time?

A) Large timescales prevent speciation from occurring.
B) Large timescales can accelerate the mutation rate, leading to greater changes.
C) Large timescales actually lead to increasing stability of the genetic material and more consistent gene expression.
D) The large span of Earth's history allows time for large changes in body form to occur; also, biologists were unaware of the underlying genetic mechanisms that can produce rapid change.