Answer:
the variable cost of adding an additional customer is very low
Explanation:
Economies of Scale is reduction in average cost of output , when the quantity of production increase. In short run, fixed cost staying in place (same), it lowers down the average variable cost of production.
It happens due to better specialisation of labour & machine, bigger & cheaper input orders, better managed supply chain & supplier connection etc. It leads to firm attaining competitive advantage in their industry.