1.选C.此句中what=a place that/which.
先行词+that/which=what.
2.选A.意思是我的学习进展得很好,但是并没有像我希望的那样顺利.此处though有不过、但是的意思.
3.选C.It is certain that...是固定句式,此时不可用sure替代.some word是指某个单词.
另附：Some与certain的区别
1.Some侧重说话人对所述之人或事不明确或一无所知,它只和单数名词连用,其前没有不定冠词.例如：
Some person called on you when you were out.你出去时有个人来拜访你.
There must be some reason for what he’s done.他这么做一定有某种理由.
2.Certain多表示说话人对所述之人或事或知之不详或知而不言,它与单数名词连用时,其前必须加不定冠词.另外,表达此意时,certain还可以同复数名词连用,表示“某些”.例如：
A certain person telephoned you while you were out.你出去时有个人给你打过电话.
For certain reasons I’ll be unable to attend the meeting.由于某些原因我不能参加此次会议.
3. 在单数普通名词前,a certain和some通常可以互换.在专有名词前通常用a certain,不用some. Some 可以和or…,or other,or another连用,表示未知的对象,而(a)certain没有这样的搭配.如：
A certain Smith is waiting at the gate.有个叫史密斯的人正在门口正等着你.
He used to work at some/a certain place in Guangzhou.他过去在广州某个地方工作.
She won a competition in some newspaper or other她在某个报纸上获过奖.