Final answer:
To round to the nearest thousandth before the addition, round each number to the nearest thousandth and then add them together.
Step-by-step explanation:
To round to the nearest thousandth before the addition, we look at the thousandths place of each number.Here we need to round each number to the nearest thousandth individually and then add them. 1234.5674 rounded to the nearest thousandth is 1234.567 and 45.67843 rounded to the nearest thousandth is 45.678. Then we add these two rounded numbers together: 1234.567 + 45.678 = 1280.245.
Therefore, the result of adding 1234.5674 and 45.67843 if you round to the nearest thousandth before the addition is 1280.245, which is option C.