An Electron Microscope Must Place the Specimen in a Vacuum

An electron microscope must place the specimen in a vacuum within a sealed chamber. Why?

A) Electrons are particles-there are also particles in air. Without a vacuum, the electrons would strike and be scattered by the atoms/particles within the air.
B) Because electrons are highly radioactive, and the chamber must be completely sealed to prevent them from escaping and contaminating the lab area.
C) This is the method for fixing the specimen to the slide grid for an electron microscope. Without fixing the specimen by vacuum, it would slide off and we wouldn't be able to visualize it.
D) All of the above are true.