Answer:
a
Explanation:
Cells are the functional units of life, this means that all living organisms are made up of cells. In multicellular organisms, cells are organized into complex structures such as tissues, organs and systems. A tissue is made up of similar cells which have the similar function. An organ is made up of several tissues while a system is made up of related organs. Example of a tissue is blood which is made up of similar cells such as red blood cells and white blood cells.
D, because all of the other options say that humans do not cause it, or that it is based on false data.
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