Answer:
A. Whenever the population has increased, steel consumption has increased as well.
Explanation:
Based on the graph of US population and steel consumption, what could have led to the increase in steel consumption seen on the graph is that whenever the population has increased, steel consumption has increased as well.
A critical look at the graph, you will discover that the population and the steel consumption are moving upwards (i.e they are increasing). It's seen that as the population increases, steel consumption increases. This is true because as the population increases, people are building houses, more transportation systems that require steel are being manufactured, more household utensils that are steel products are being fabricated etc; therefore the consumption of steel increases.
