Question

which reason best explains why dead specimens must be used with transmission electron microscopes?
electrons pass over the specimen.
the lights that are used are harmful to the specimens.
specimens are placed in a vacuum.
the image that is produced is two dimensional.

Explanation:

Transmission electron microscopes require specimens to be placed in a vacuum. Living specimens cannot survive in a vacuum environment, so dead specimens must be used. The other options do not directly explain why specimens must be dead. For example, electrons passing over the specimen (scanning electron microscopes use electrons passing over, but this isn't the key reason for dead specimens in transmission electron microscopes). The lights used in electron microscopes are not the issue (they use electrons, not light in the traditional sense like compound microscopes). The two - dimensional image is a characteristic of the image produced, not a reason for using dead specimens.

Answer:

Specimens are placed in a vacuum.