Aiden is a spice trader. The price he charges for an order of cumin (in dollars) as a function of the size of the order (in kilograms) is graphed.

What is the approximate average rate at which the total cost increases, as the order size increases from 100 to 600 kilograms?

Options:

1: $1.50
2: $1.80
3: $2.10
4:$2.40