As far as I know, shopping online has many advantages, _____ the most important one is convenience.[ ]

A. during which  

B. of them

C. of which  

D. of whom

下列关于直喉镜和弯喉镜的描述哪项是正确的

A．弯喉镜的操作必须在标准头后仰位下进行

B．直喉镜不必过度后仰头部

C．直喉镜直接挑起会咽显露声门

D．直喉镜和弯喉镜的操作没有什么不同

E．弯喉镜必须以下门牙作为支撑挑起会咽显露声门