Darwin Developed His Theory of Natural Selection Despite Having an Incomplete

Darwin developed his theory of natural selection despite having an incomplete and/or inaccurate knowledge base from which to work.Which of the following were problems for Darwin?

A) the source of variations in populations
B) an understanding of inheritance patterns from one generation to the next
C) The accepted age of the Earth was far too young to allow for the gradual changes Darwin envisioned.
D) All of the above.