2006.5 SAT 语法 characteristic可数吗

SAT2006.5真题语法问题
1.The candidate called for medical insurance reform,but to me he seemed less interested in the plight of the uninsured citizens than in whether enough is registered to vote.
enough is怎么改啊?
2.Because his experience in the naval medical corps had been rewarding,Bob applied to medical school after he was discharged from the navy
rewarding为什么是对的
3.Africa's Kanem empire,after enduring for over a thousand years,is believed to have fallen into decline when trade centers shift outside its boundaries.
shift有什么问题啊?