29. What does the passage mainly discuss? (A) The detail seen through a microscope (B) Sources of illumination for microscopes (C) A new kind of microscope (D) Outdated microscopic technique

30. According to the passage, the invention of the visible-light microscope allowed scientists to (A) see viruses directly (B) develop the electron microscope later on (C) understand more about the distribution of the chemical elements (D) discover single celled plants and animals they had never seen before.