How Were North and South Yemen Able to Form an Agreement

How were North and South Yemen able to form an agreement and become the united Republic of Yemen?

A) Terrorist attacks had plagued both nations and so in order to improve things, they united their efforts and their nations.
B) The need to effectively exploit discoveries of oil in the ill-defined border region between the two impoverished countries.
C) They were both experiencing shortages of food and water, to be more effective they joined forces to help each other and solved this crisis.
D) A terrorist group gained power in both countries and formed an Islamic Caliphate by combining the two nations.