题干
It was not until the subprime loan crisis（次贷危机） ____ great damage to the American financial system that Americans ____ the severity of the situation．
A：caused, realized
B：had caused, realized
C：caused, had realized
D：was causing, had realized
答案(点此获取答案解析）
B
，
是二次函数，若
的值域是
，则