Fact 1: Viruses are so small that they can be seen only with an electron microscope.

Fact 2: Some smallpox viruses are maintained in laboratories.

Given the two facts above, answer the question "What is so small it can be seen only with an electron microscope?" with the following options: 
- all invertebrates
 - humans
 - bacteria
 - alveoli
 - acetic acid
 - orchids
 - smallpox
 - planets


 

       smallpox  