We have been good friends ____ we joined the same ping-pong team. [ ]

A. after

B. before

C. since

D. until

你所在的生产厂钢材产品存在哪些缺陷（答4种以上）？哪些是钢质缺陷？哪些是操作缺陷？请简述其产生的原因。