4.It is reported that the old coins _____ under the earth for about 500 years by the time they were discovered.

4.It is reported that the old coins _____ under the earth for about 500 years by the time they were discovered.
A.had buried \x05\x05\x05B.had been buried
C.buried \x05\x05\x05D.were buried
这个怎么选,为什么