上面的都不正确,
首先 ____ at the conference.在句中做定语修饰 proposals 而且表示被动意义．
第二,非谓语做定语并表被动的有三种形式,1,to be done,2,done,3,being done .但他们表示的时间不同,
to be done,表示将来;done,表示已经过去或没有确切的时间,being done 表示正在进行.例如
He showed us the model of the bridge to be built next year.明年建的桥-将来
He showed us the model of the bridge being built now正在进行
He showed us the model of the bridge built last year.过去
I do not like letters written in pencil.没确切时间
另外;.having been discussed 现在分词的完成式是不能做定语的