Fact 1: Cells vary in size as well as shape, but all cells are very small.

Fact 2: An embryo is so small that it requires a microscope in order to be seen.

Given the two facts above, answer the question "What requires a microscope in order to be seen?" with the following options: 
- fungi
 - cells
 - mildew
 - gasoline
 - fungus
 - matter
 - telescopes
 - sunlight


 

  cells       