The transmission electron microscope is most useful for
looking at surface details of a virus
viewing a cell's mitochondria
seeing animal tissue that is visible to the naked eye
studying the structures of a live paramecium

i think it might be the first one

Viewing a cell's mitochondria.  

A transmission electron microscopy is a microscopic technique in which a beam of electron is transmitted through a specimen to form an image. It magnifies the structure of the specimen with high resolution as compared to light microscopes. It can be used to examine the internal structure of the cell. Transmission electron microscopy can be used to study the organelles in the cells. Hence, it can be used to view a cell's mitochondria.

oxygen is the first element in group 16 (via) of the periodic table. the periodic table is a chart that shows how chemical elements are related to each other. the elements in group 16 are said to belong to the chalcogen family. other elements in this group include  sulfur, selenium, tellurium,  and  polonium.  the name chalcogen comes from the greek word  chalkos,  meaning "ore." the first two members of the family, oxygen and sulfur, are found in most ores.

oxygen is by far the most abundant element in the earth's crust. nearly half of all the atoms in the earth are oxygen atoms. oxygen also makes up about one-fifth of the earth's atmosphere. nearly 90 percent of the weight of the oceans is due to oxygen. in addition, oxygen is thought to be the third most abundant element in the universe and in the solar system.