About a question in Longman preparation course for the TOEFL test

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
© 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.

TOEFL listening lectures: A lecture from an American History class