龋病是牙硬组织发生慢性进行性破坏的一种疾病，龋病按照进展情况分为（）

单项选择题In which of the following techniques is a film holder a necessity

单项选择题Which one of the following may appear on a periapical radiograph of the upper third molar area（）

A．external oblique ridge
B．mylohyoid ridge
C．coronoid process
D． condyle

单项选择题Which of the following projections images the crowns of both maxillary and mandibular teeth on the same film and is helpful in diagnosing interproximal dental caries

单项选择题Which one of the following anatomy should not appear on the periapical radiograph of the lower central incisor

单项选择题Periodontal ligament space appears as a radiolucent space between the tooth root and the （）.

A．alveolar crest
B．lamina dura
C．cancellous bone
D． trabecular bone

单项选择题Which one of the following anatomy is not usually apparent radiographically（）

A．enamel
B． dentin
C．cementum
D．pulp cavity