________hard before, Tom is afraid of failing in the exam.

A．Having been worked

B．Not to have worked

C．Having never worked

D．Never have worked

下列不是口腔颌面部感染特征的是

A．口腔、颜面及上辱吸道感染，可顺相应淋巴引流途径扩散
B．儿童较成人更易发生腺源性感染
C．口腔颌面部感染可借血液循环扩散至邻近间隙
D．“危险三角区”的感染处理不当可造成上呼吸道梗阻
E．口腔颌面部组织抗感染能力较其他组织为强