The correct answer is "A. carbon, hydrogen, oxygen and nitrogen".
Scientists believe that out of the 116 known elements, only 25 are used
by mammals (and therefore by humans). Out of the 25 elements that are
believed by scientists to support human life, only four of these
elements - carbon, oxygen, hydrogen, and nitrogen - make up about 96% of
the human body and are found in the basic structure of all biochemical
molecules within the human body. Because of this, carbon, hydrogen,
oxygen, and nitrogen are considered "the four essential elements"
supporting human life. However, it is also important to keep in mind
that although many of the other 25 elements present in the human body
are required in only small amounts, they play an important role in
keeping the body functioning correctly, as well.
Sample Response: The first stage of cellular respiration occurs in the cytoplasm. This is where glucose is broken down into smaller molecules. A small amount of energy is also released. The second stage occurs in the mitochondrion. This is where the small molecules combine with oxygen to produce water, carbon dioxide, and a large amount of energy.
The complex eukaryotic cell ushered in a whole new era for life on Earth, because these cells evolved into multicellular organisms. Evidence supports the idea that eukaryotic cells are actually the descendents of separate prokaryotic cells that joined together in a symbiotic union.
