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

高等级公路的沥青路面一般应采用（）。

A．单层

B．单层或双层

C．只能是双层

D．双层或三层式