【名词&注释】
青少年(adolescent)、牙龈炎(gingivitis)、高发期(peak period)、牙萌出(tooth eruption)、牙周膜厚度(thickness of periodontal ligament)
[单选题]正常牙周膜厚度(thickness of periodontal ligament)为
A. 0.15~0.38mm B. 1mm C. 2mm D. 6mm E. 4mm