Identify the correct inverted sentence structure:

A. Never before the legal field has faced such pressure.

B. Rarely the regulators consider structural reforms.

C. No sooner had the regulation been changed than investments increased.

D. Only after ten years the system was modified.

核心考点：倒装句。解题思路：否定词短语置于句首需部分倒装。正确选项C("No sooner had...")符合倒装规则，错误选项A/B/D均缺失助动词倒装。易错点：未识别否定词触发倒装的语法规则。

正确答案：C