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

OA is A
I need the help of an expert here to give me reasons on A is correct, B gives some hint too. Thanks in anticipation.