1. (10 points) Two bags of flour weigh the same. From the first bag, $\frac{1}{3}$ is used, and from the second bag, $\frac{1}{3}$ kilogram is used. The remaining flour ( )
A. The first bag is heavier
B. The second bag is heavier
C. Both bags weigh the same
D. It cannot be determined which bag is heavier
【Analysis】It depends on the actual situation, related to the original weight. Categorize and discuss: greater than 1 kilogram; equal to 1 kilogram; less than 1 kilogram, and the conclusion can be drawn.

【Solution】Solution: It depends on the actual situation, related to the original weight.
When greater than 1 kilogram, the first bag uses more than $\frac{1}{3}$ kilogram, so the second bag has more left,
When equal to 1 kilogram, the remaining weight is the same,
When less than 1 kilogram, the first bag uses less than $\frac{1}{3}$ kilogram, so the first bag has more left,
Therefore, the answer is: $D$.