The problem will appear __a different welfare system will be

The problem will appear __a different welfare system will be created.A unless.B whether.C until.D if.正确答案是b,为什么不选a

这是同位语从句,亲,由whether引导,其后面的部分是problem 的内容.
也许你会有二楼同样的疑问：“同位语从句一般在名词后面吗?”
那是因为这是一个分隔式同位语从句,就是先行词和引导词之间有插入其他成分的情况.
给你个例子看看：
An idea（ came to him ）that he might write to her toask more information about the matter.
处理方法是括号中的部分可以放到句子末尾.
小结一下：
英语中,同位语从句一般是紧跟在与其同位的词后面的.但是,由于表达不同意义的需要,同位语从句可以被句子其它成分隔开,这种从句就叫做“分隔同位语从句”.分隔同位语从句也是英语中比较常见的一种语言现象,现将这种现象简单归纳如下.
　　　　一.被状语隔开
　　　　1.条件状语
　　　　I had the feeling- if one can actually receive feeling from liver Barrett III -that he meant to give me the money.假如一个人确实能够从奥利弗.巴雷特三世那儿得到感觉的话,那么,我觉得他打算给我钱.
　　　　2.程度状语
　　　　We have no idea at all that he has gone.我们一点儿都不知道他已经走了.
　　　　After that they gave their father the bags of gold and there is no doubt at all that they all lived together happily ever after.接着,他们把那几袋金子给了父亲,毫无疑问,从那以后,他们就幸福地生活在一起了.
　　　　3.方式状语
　　　　We ' ve just heard a warning on the radio that a hurricane is likely to come.我们刚刚从收音机里听到警报说,飓风可能就要来到了.
　　　　4.范围状语
　　　　There is a popular belief among parents that schools are no longer interested in spelling.学生家长有一个普遍的看法,那就是学校不再注意拼写了.
　　　　Though Mrs Bussman knew this story quite well,she thought there was a chance in a million that she might be right.尽管巴斯曼太太对这件事很熟悉,但她还是觉得有万分之一的可能性：她是对的.
　　　　5.时间状语
　　　　What they really value among the gifts that we bring to them is intoxicating liquor,which enables them,for the first time in their lives,to have the illusion,for a few brief moments ,that it is better to be alive than dead.他们喜欢的是我们给他们的令他们头号重脚轻的烈性酒,因为这能使他们平生以来第一次产生短暂而美好的幻觉：生比死好.
　　　　6.地点状语
　　　　There ' s this opinion from outside China that Shanghai should not be considered for hosting the Expo because Beijing has already won the bid for the 2008 Olympics.国外有这种看法,认为不应该再考虑由上海举办世博会,因为北京已经赢得了2008年的奥林匹克运动会的主办权.
　　　　Tiger once expressed his wishes in an advertisement that he could play anywhere he wanted.泰格曾在一次广告中表达了他的愿望,希望能在他喜欢的任何地方打球.
　　　　二.被定语隔开
　　　　Far from the truth are the words given by the observer that John was not murdered.约翰不是被谋杀的,目击者所说的这些话远远不符合事实.
　　　　The night before Mother ' s Day there were broad hints passed across the table from the two older children that tomorrow morning I would be happily surprised by what they had gotten me as presents.母亲节前的那个晚上,坐在桌子对面的两个大点儿的孩子明显地暗示,次日早上我会对他们送给我的礼物感到惊喜.