YJ19供料系统平准器劈刀盘圆周上有（），是保证在烟支两端烟丝密度比烟支中间的密度大，两凹槽中心间的圆周距离等于烟支长度。
参考答案：凹槽
简单描述用户暗管内穿放蝶形光缆方法
Inflatable liferafts shall be serviced at an approved servicing facility every 12 months or not later than the next vessel inspection for certification. However，the total elapsed time between servicing cannot exceed（）.
A.12 months
B.15 months
C.17 months
D.18 months
