Viruses are so small that they can be seen only with an electron microscope, and some smallpox viruses are maintained in laboratories. Given these facts, what is so small it can be seen only with an electron microscope among the following options:
- all invertebrates
 - humans
 - bacteria
 - alveoli
 - acetic acid
 - orchids
 - smallpox
 - planets


 

       smallpox  