Which contribution by Antonie van Leeuwenhoek most directly advanced the development of cell theory?

• A. He was among the first to observe and describe living microorganisms ("animalcules"), including bacteria and protozoa, using improved single-lens microscopes.
• B. He concluded that all living organisms are composed of one or more cells and that the cell is the basic unit of life.
• C. He first identified the cell nucleus and proposed that it controls cellular activities.
• D. He formulated the principle that all cells arise from pre-existing cells ("omnis cellula e cellula").

Answer: (A)