B. Because even with many issues that plagued the Roman Empire economically and politically, By the later years of the empire the Roman military was mainly comprised of trained Barbarian soliders who were inferior to earlier Roman soliders causing a significant decrease in the Roman Empires's Military capablities so many barbarian tribes that began to grow such as the Huns, Vandals, Goth etc. Put immense pressure on the Empire, driving them into the ground economically and causing many issues within the government and eventually being sacked, thus marking the end of the Roman Empire. Sorry for the wall of text but hope this helped :)
The final loss of Jerusalem for European forces can be considered to be between 1270 and 1272. In the year 1270, Louis IX of France who lead the (arguably called) Eight crusade died on the shores of modern day Tunisia. This had effectively ended the Eight crusade and also led Henry III of England's son Edward to sail to Acre and try to reclaim the city. He was, however, unsucessful and had to return back to England. His demise marks the ending of the Ninth crusade which lasted from around 1271 to 1272. After this crusade no major attempt from Europe was made again to reclaim Jerusalem or any other city captured by the Crusaders in previous crusades.
A. It became a trading center due to its location and size.
