In the history of cell biology, what was Antonie van Leeuwenhoek's key contribution that helped advance ideas underlying cell theory?

• A. He concluded that new cells arise only from pre-existing cells based on studies of cell division.
• B. He discovered the nucleus and described it as a defining feature of eukaryotic cells.
• C. He improved single-lens microscopes and was among the first to observe and describe living microorganisms (e.g., bacteria and protozoa), which he called "animalcules."
• D. He proposed that all living organisms are composed of cells and that the cell is the basic unit of life.

Answer: (C)