A market research company wants to survey commercial building managers as to whether they plan to install solar power on the roof of their building in the next five years. The profitability of such installations is higher in provinces with higher prices for purchasing electricity from the electric distribution company, that is, from the public electricity grid. They select a random sample of building managers from provinces with high electricity prices, another random sample from those with medium electricity prices, and a third random sample from those with low electricity prices. What kind of sampling technique have they used? Give reasons for your answer.