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 $3.44 per gallon when filling several tanks). The exchange is about the same price. Keep in mind this comparison is as if the exchange gives you a FULL tank.
Just as a note to all of you. If you trade in you are only getting 13 - 15lbs of propane in the 20lb tank for about 18.00 - 21.00$. It cost me 12.00 for 20lbs to have my tanks refilled. Most rental places refill tanks as well as Ace Hardware.