Pick the two sentences in order that fit best between sentence S1 and S4:
S1. Shops and homes in select areas of Maharashtra will soon have to pay in advance for the electricity they use.
P. This follows the Maharashtra Electricity Boards approval for introducing prepaid meters.
Q. The states' electricity distribution company will install 25,000 prepaid meters to begin with.
R. The electricity meters will be installed free of cost in those shops and residences where usage is low.
S4. Customers using these meters will have to buy electricity using a card which can be recharged when the meter shows low credit.