请教02-10的第6题啊,谢谢

Newfish

6. Ohio, the center of_____ the Hopewell culture, has the greatest concentration of ancient burial mounds in the United States.  

(A)  called

(B)  what is called

(C)  that is called

(D)  is called

答案是B,能帮忙解释一下吗,谢谢啊

shaohan2000

what is called the Hopewell culture   

构成名词性短语放在of前当然才对

Newfish

thanks