Answer
1 kilometer is longer than 1 millimeter
Work Step by Step
1 kilometer is equivalent to 1000 meters. 1 meter is equivalent to 1000 millimeters.
$1km\times\frac{1000m}{1km}\times\frac{1000mm}{1m}=1,000,000mm$
Since 1,000,000 is greater than 1, 1 kilometer is larger than 1 millimeter.