Only when the European countries' financial system broke down to take measures to deal with those problems．
A. did they decide
B. they decided
C. they had decided
D. had they decided
网友回答
(答案→)A. did they decide
B. they decided
C. they had decided
D. had they decided
解析：考察倒装句，当only放句首时，后面的主句要用部分倒装句，即提前助动词或者情态动词，根据句子结构可知，主句的时态应当和when引导的时间状语从句的时间保持一致，所以选择A
句意为，只有当欧洲国家的经济体系崩溃的时候，他们才会决定采取措施处理这些问题。