You are presented with the quiz "Viruses are so small that they can be seen by what?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "viruses are so small that they can be seen only with an electron microscope" and "once the camera is tuned with the microscope, the focusing depends on the microscope lens". 

So, what's the best answer to the question?


 

 focusing a lens          