—Few pupils are as bright as he is, and he works very hard, too.

—It is no          that he always gets the first place.

A．problem

B．evidence

C．wonder

D．doubt

根据脊柱三柱概念，轻度或中度椎体压缩骨折，但后柱结构尚保持完整，称稳定性脊柱损伤。脊柱的三柱中有两柱损伤者，称力学性脊柱不稳定。椎管内存在骨和椎间盘碎片导致神经根和马尾受压者，称（）。