这类患者慢性骨髓炎经久不愈，反复发作的主要原因是
A．窦道的形成B．死骨的残留C．瘢痕组织增生D．机体抵抗力低E．细菌毒力太强
参考答案：B
__________ the students are doing their maths exercises with the help of the teacher.
A．Three score of
B．Three scores
C．Score of
D．Scores
加油站应定期检查各电气接线是否牢固，有无锈蚀。
