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In 1839 a scientist named Theodor Schwan proposed a theory that later became known as the cell theory. The theory stated the following.

1.All living things are made up of cells.
2.The cell is the most basic unit of life.

The theory at the time did not include where cells came from. In 1859, Louis Pasteur conducted an experiment that showed that living cells must come from other living cells. This idea was added to the cell theory. The other two points of the theory were not discarded, so the theory was modified. Why was the older cell theory modified and not discarded?

A.
Scientific theories cannot be discarded after a certain period of time has passed.
B.
The first two ideas of the theory cannot be changed since they are scientific laws.
C.
The first two ideas of the cell theory were still supported by evidence.
D.
Scientific theories require the use of some older scientific knowledge in order to be valid.