How to add each line for Column A and Column B and place the answer in Column C for each line?

edit

In C1 put:

=SUM(A1+B1)

Then click-and-drag down Col C the "handle" in the lower right corner of the C1 cell.

The column will populate correctly, i.e.:

=SUM(A1+B1)
=SUM(A2+B2)
=SUM(A3+B3)
... etc.

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