Christian Missions in Persia. Christianity was introduced in Persia in the Parthian period, and several bishoprics were established there (Latourette, II, pp. 263ff.).
You will likely pay more for the exchange, even if your tank is completely empty. An empty 20 lb tank will hold about 4.7 gallons of propane. That is $18.47 at $3.93 per gallon, which is the low quantity price (I just paid $3.44 per gallon when filling several tanks). The exchange is about the same price. Keep in mind this comparison is as if the exchange gives you a FULL tank.
Well, if your home is in Nebraska, filling a 500-gallon propane tank (300 gallons if you are at 20% capacity and fill it to 80%) will cost you $562 at current average propane prices per gallon, compared to $397 last year. That is $165 more expensive.