应用下列哪种栓塞材料后，侧支循环难以建立（）

A.不锈钢圈

B.明胶海绵

C.微球

D.血凝块

E.球囊

Which one of the following may appear on a periapical radiograph of the upper third molar area

A．external oblique ridge

B．mylohyoid ridge

C．coronoid process

D． condyle