he disappeared last month,--------- we have heard nothing of

He disappeared last month, _________ we have heard nothing of him.
A when B after which C from which D since when
你说的是对的,这确实是个非限制性定于从句.最佳答案选D,但是这是一句很别扭的英语.
1. 非限制性定语从句中,which这个引导词可以指先行词,也可以指前面整个句子所陈述的事情.
“如果选when就成了since in which（?）”并不一定是这样的.我们说,where 常可以用"介词+which"来替换,但是这里的“介词”不一定就是in.如：
He climbed out of the chimney, where he had spent two hours.
=He climbed out of the chimney, (in which) he had spent two hours.（where 在烟囱里）
He fell down the tree, where he could see the star on the stage.
=He fell down from the chimney, (on which ) he could see the star on the stage. (where在烟囱上)
同理,when=介词+which的时候,不一定是in,有可能指的是from which,during which
而且,咱们一般不这样替换过来,而且也不是所有的都能替换,尤其是when.这里的when就指前面的时间last month
2. A答案里面when不引导非限制定于从句.B的after which 这里的which可以理解成“他上个月消失”这件事情.意思上,是正确的,但是后面用的完成时,since常用在完成时.C不对,
再问： 关键是这里since不是介词 而when在这里是个名词对不对？
再答： when rel adv 关系副词 (used after time, day, month, etc 用於time﹑ day﹑ month等之後) at or on which 在那时; 其时: Sunday is the day when very few people go to work. 星期日是没什麽人上班的日子. * There are times when I wonder why I do this job. 有时候我也不明白我为什麽要做这个工作. * It was the sort of morning when everything goes wrong. 那天上午就是那样, 事事不顺心. at which time; on which occasion 在那时; 当时; 当场: The Queen's last visit was in May, when she opened the new hospital. 女王上次来访是在五月份, 她那时主持了这座医院的落成典礼.
再问： when 在这里不是引导定语从句的吧？
再答： 牛津词典： when adverb, pronoun, conjunction pronoun what/which time: Until when can you stay? ◆ 'I've got a new job.' 'Since when?' since preposition, conjunction, adverb 1. preposition (used with the present, perfect or past perfect tense) from a time in the past until a later past time, or until now: She's been off work since Tuesday. ◆ We've lived here since 1994. ◆ I haven't eaten since breakfast. ◆ He's been working in a bank since leaving school. ◆ Since the party she had only spoken to him once. ◆ 'They've split up.' 'Since when?' ◆ That was years ago. I've changed jobs since then. Help Note: Use for, not since, with a period of time: I've been learning English for five years. ◆ I've been learning English since five years. 2. ~ when? used when you are showing that you are angry about sth: Since when did he ever listen to me? 你可以理解为固定用法，这个第二点是单独作为一条出现在释意里面的。 我认为这里的since是个介词，这里的when是个代词，你要说when引导的是个从句的话，那么他就是个宾语从句。 理解成固定用法吧~