Scientists believe that the earth's early atmosphere lacked gas because photosynthetic organisms did not yet exist.
a) hydrogen
b) nitrogen
c) oxygen
d) sulfur

How do diseases caused by bacteria and diseases caused by viruses react to antibiotics?

Why are fossils not found in igneous rocks? igneous rocks are made from cooling of lava or magma. igneous rocks are found too deep underground. igneous rocks are too dark in color to contain fossils. igneous rocks are too dense to contain fossils.

Which of the following does not make up ground substance of connective tissue? hyaluronic acid elastic fibers glycosaminoglycan proteoglycan