【名词&注释】
动脉瘤(aneurysm)、脑震荡(brain concussion)、牙周膜厚度(thickness of periodontal ligament)
[单选题]正常牙周膜厚度为()
A. 0.15~0.38mm B. 1mm C. 2mm D. 6mm E. 4mm