Final answer:
The answer is 3,000 km is bigger than 3.5 km.
Step-by-step explanation:
When comparing 3.5 km and 3,000 km, we need to convert them to the same unit. Since they are both in kilometers, we can compare them directly. Since 3,000 is greater than 3.5, we can conclude that 3,000 km is bigger than 3.5 km.