In the history of cell biology, what was Zacharias Janssen's key contribution that helped enable later discoveries leading to cell theory?

• A. He is often credited with inventing (or helping invent) the first compound microscope, allowing magnified observation of tiny structures.
• B. He discovered and named the cell after observing cork under a microscope.
• C. He proposed that all living organisms are composed of cells and that cells arise from pre-existing cells.
• D. He first observed living cells ("animalcules") and described their motility in pond water.

Answer: (A)