In reply ______your order of December 15th, we regret ______say

In reply ______your order of December 15th, we regret ______say that we do not have______stock any such articles as you describe.
【正确答案】：to,to,in。in reply to sth．表示“对……的回复”，此时的to为介词；regret to do sth．表示“很遗憾做某事”；have sth. in stock表示“有……现货”。