GMAT语法：if 虚拟语气的主从句是否需要配对使用？

loveoaple

以下是一道很经典的GMAT语法题，答案选A

Developing nations in various parts of the world have amassed $700 billion in debts; at stake, should a significant number of these debts be repudiated, is the solvency of some of the world’s largest multinational banks.

(A) should a significant number of these debts be repudiated, is
(B) should a significant number of these debts be repudiated, are
(C) should they repudiate a significant number of these debts, are
(D) if there is a repudiation of a significant number of these debts, would be
(E) if a significant number of these debts will be repudiated, is

我的疑惑在于“If虚拟语气”是否必须配对使用？就A选项来说，从句 should a significant number of these debts be repudiated 为对将来的虚拟，那么对应的主句  the solvency of some of the world’s largest multinational banks is at stake 中的 is 应该改成should be 才对啊！为什么是 is 呢？

请高手指点，感激不尽~~~