You are presented with the question "What is so small it can be seen only with an electron microscope?" and the following answer choices: 
- all invertebrates
 - humans
 - bacteria
 - alveoli
 - acetic acid
 - orchids
 - smallpox
 - planets

Now knowing that viruses are so small that they can be seen only with an electron microscope and some smallpox viruses are maintained in laboratories, choose the best answer.


 

       smallpox  