Africa and Quarantine

So i saw Prometheus again with my wife yesterday. Somethings that I realized the second time around... When David is scoping out the control room(room with the Engineer in Cryo) everytime it shows Earth it shows Africa Specifically. So my take is that they were going to use the bio weapon on a remote set of land on Earth which would be Africa. After all Africa is its own Continent and if the bio weapon got out of control, it would be easy to blow up the continent to contain the weapon.(Unless Xeno's swim...which they do according to Resurrection..but we dont know their limits. So thats the first part. Because the Engineers Bio weapon was unleashed on the planet and ship, it looks like they were going to all be in quarantine until the bio weapon died off. This would explain why there was still one engineer left. He was there to make sure that everything had died off before he could return to his home planet. Maybe the sequel would work around this somehow...the Engineers leave enough time for the Xeno's to die off and then come back..only to find out that Humans came back and messed things up. Thus causing a huge battle between Bio weapons and Engineers. Also as a fun side note...in civilizations when a place was put under quarantine they would have some kind of sign, poster, monument that would state this. Prometheus has a monument. Remember when the wind storm happens and you see a face built into the ship that looks like a person screaming...thats the sign. There were several other ships according to David. Its possible that one of the other ships still had Xeno's on them. So its possible that the Xeno we saw burst out of the Engineer becomes the ruler or queen over the other Xeno's...since they would be supreme over the other Xenos(thanks to several forms of mutation) After all the xeno's only like to come out at night, and if the Xeno's set up shop in one of the several other ships then the crew wouldnt see them. The only people whom could possible see them were shut in the Black Goo room(with the earth worm snake like things) We know they were shut in because when the crew goes looking for them the next day the door has to be re-opened.