【名词&注释】
体外受精(in vitro fertilization)、根管治疗术(root canal therapy)、牙颈部(dental neck)、变异干髓术(modified pulp mummification)、同源人工授精(artificial insemination by husband)、异源人工授精(artificial insemination by donor)、牙髓失活(pulp devitalization)、无性生殖(asexual reproduction)、固定桥基牙(fixed bridge abutments)、经常出现(often appear)
[单选题]固定桥基牙(fixed bridge abutments)临床冠根比的最低限度为
A. 1:1
B. 1:2
C. 1:3
D. 2:3
E. 3:2