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

甲有一价值100万元的房屋，甲向乙借款10万元的时候以该房屋依法定程序设定抵押，之后甲又向丙借款20万元，也是以该房屋依法定程序设定的抵押，关于该抵押行为，下列说法正确的是()。

A.因为不可以在同一抵押物多次设定抵押，所以之后甲对丙设定的抵押是无效的

B.由于没有超出100万元的房屋价值，因此可以再次抵押

C.甲不得就同一房屋进行两次抵押

D.甲对丙的抵押行为合法，对乙的抵押行为不合法