上下颌双侧后牙缺失，上颌第一双尖牙牙冠颊面的长短应以什么牙牙冠作为标准()
A．中切牙
B．侧切牙
C．尖牙
D．磨牙
E．以上均不正确
参考答案：C
从一个新鲜的紫色洋葱表皮细胞来看，组成原生质层的是
①细胞壁 ②细胞膜 ③细胞质 ④液泡膜 ⑤液泡 ⑥细胞液[ ]
A．①②③
B．②③④
C．③④⑤
D．②④⑥
The story ____ by the famous writer is ____ at the factory now. [ ]
A. wrote, printed
B. written, being printed
C. write, being printed
D. written, printed
