Only when________the data again________that there was a mistake. [ ]  

A. they had studied；did they realize

B. they had studied；realized they

C. had they studied；did they realize

D. had they studied；realized they

过桥走线架（槽道）跨于机房两侧主走线架（槽道）之间时，应采用连接件与机房两侧的连接铁和主走线架（槽道）加固.当加固点间距超过2000mm时，应采用（）.

A.临时立柱加固措施

B.固定立柱加固措施

C.吊挂加固措施

D.可以不采用措施