About a question in Longman preparation course for the TOEFL® test

[Nhunguyen]

Hi,
I am preparing for TOEFL test. And there is one question that makes me confused.
Choose the best answer to fill in the space
"According to the World Health Organization, _____ there to be an outbreak of any of the six most dangerous diseases, this could be cause for quarantine."

(A) were
(B) they were
(C) there were
(D) were they

To me, I don't know which answer is correct. However, according to the textbook, the answer is (A).
I don't know the reason for it. This question is so strange to me. Somebody can help me to explain the structure of this question. Why they use "were + there + to be"? I tried to look up inside some grammar book, but in fact, I cannot analyze the structure, so that, I cannot find out this point of grammar.
Please help me.
Thanks in advance.