2008年10月SAT一道语法题求解释!

2008年10月SAT一道语法题求解释!
改进句子的
(If there had been) a power failure,the hospital will run on electricity from its own generators,which can operate for 200 hours.
A.If there had been
B.If there were
C.Had there been
D.Should there be
E.There has been
D是正确答案我能懂,但是为什么B不对?虚拟语气对未来表推测不是可以用If+主语+were的么?T