What made Crusader states, such as the Kingdom of Jerusalem, vulnerable?

A.They had no means of provisioning themselves.
B.They had no defenses.
C.They were entirely surrounded by Muslim lands.
D.They were hard-hit by plagues endemic to the region.

Question

What made Crusader states, such as the Kingdom of Jerusalem, vulnerable?	
	
A.They had no means of provisioning themselves.
B.They had no defenses.
C.They were entirely surrounded by Muslim    lands.
D.They were hard-hit by plagues endemic to the region.

anonymous · Answer

All the given options make some sense: The kingdom, itself a desert, was surrounded by hostile territories  so provisioning was a major issue. Also, when their soldiers were away on a crusade the kingdom was left with very limited defenses. They were surrounded by muslim lands and plagues in those times are recurrent. 
The most important among all  these is the location of the muslim states in their neighborhood, as their crusades were predominantly against  muslim occupation of the 'holy land' which in turn made a bunch of hostile neighbors even more hostile. So any attacks from the muslim states would come from all sides and there would be limited options for defence with almost no scope for escape.