A .(50S细纱锭速×某种纱捻度×某种纱支)/某种纱细纱锭速×50S纱捻度×50
B .(某种纱细纱锭速×某种纱捻度×某种纱支)/50S细纱锭速×50S纱捻度×50
C .(某种纱细纱锭速×50S纱捻度×某种纱支)/50S细纱锭速×某种纱捻度×50
D .以上都不对