In the history of cell biology, what key contribution did Rudolf Virchow make to the cell theory?

• A. He proposed that all cells arise from pre-existing cells ("Omnis cellula e cellula").
• B. He developed the first widely accepted model of the fluid mosaic structure of the plasma membrane.
• C. He first observed and named "cells" in cork using a light microscope.
• D. He concluded that plants are composed of cells and that the cell is the basic unit of plant structure.

Answer: (A)