You are presented with the quiz "What of the following can only be viewed by illuminating electrons? " 

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 "objects to be viewed are illuminated by electrons in an electron microscope". 

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


 

       viruses    