Amy looks sad.Could you tell me（）that prevents her from being as happy as before?A.what it is B.it is what C.how it is D.it is how 选什么,为什么?

Amy looks sad.Could you tell me（）that prevents her from being as happy as before?A.what it is B.it is what C.how it is D.it is how 选什么,为什么?
Amy looks sad.Could you tell me（）that prevents her from being as happy as before?A.what it is B.it is what C.how it is D.it is how 选什么,为什么?

Amy looks sad.Could you tell me（）that prevents her from being as happy as before?A.what it is B.it is what C.how it is D.it is how 选什么,为什么?
我认为选择B,因为it is...that...为强调句型,这一部分可省略,则宾语从句为what prevents her from being as happy as before.意为什么让她不再像以前一样快乐了.