1 Having a trip abroad is certainly good for the old couple ,but it remains____whether they will enjoy it.
A to see B to be seen C seeing D seen
2 There is a new problem involved in the popularity of private cars_____road conditions need______.
A that;to be improved B which;to be improved
C where;improving D when;improving
3 It is said in Australia there is more land than the goverment knows_____.
A it what to do with B what to do it with
C what to do with it D when;improving
4 When I got back home I saw amesssage pinned to the door_____"Sorry to miss you;will call later"
A read B reads C to read D reading
5 Rober is said ____abroad,but I don't know what country he studied in.
A to have studied B to studying
C to be studying D to have been studying
请详解一下

1.答案选b,it remains to be seen...这是固定句式，意思是：...仍然需要进一步观察，it是形式主语，真正的主语是seen后的句子，我们理解的时候也可以这样来考虑：remain to do 是固定的用法，意思是：仍然做某事，因为whether they will enjoy it.他们是否喜欢这件事仍然有待于被考察，事情是被考察的，是被动，所以用remain to be seen
2.答案选a,that引导同位语从句，具体解释the problem 的内容，involved in the popularity of private cars是过去分词做定语，修饰the problem,指的是涉及私家车普及的问题，公路状况需要被改善，所以用need to be improved,也可以用need improving 来表示
3.答案选c，这个高考题考察特殊疑问词+to do,it指land,do with :处理，应付，它一般与what搭配，所以what to do with it: 怎样处理土地
4.答案选d，这个高考题考察see+宾语+宾补，因为纸条被大头针定在门上，所以用过去分词，pinned表示被动，又因为Sorry to miss you;will call later"这是纸条本身的内容，read的意思就是指信息或者标牌上的内容，不是读懂的意思，是指条自己的内容，所以用主动式reading,表示看到纸条上正存在的内容
5.答案选a，由I don't know what country he studied in我们知道，studied表示过去发生的事情，现在已经不存在了，所以用to have studied,表示过去发生的事情，现在已经结束，这个学习的动作先于know发生