英语问题...在线急求 解答

Although she received a lot of money,she ___ sad because she could never see her son again.
A.carry on B.went on C.remained D.kept on答案是C 能解释下其他项的用法吗？？？
Some airline companies are now doing everyting they___ bring forward cost-saving ticketless flight.
A.can B.can to C.could D could to 答案是B。。D与B有什么不同？？？能具体说说吗？
The secretary has a lot of things to ___ in the offic since she has been away for quite a few days.
A,take up B. make up 答案是B，A为何不行？？？

Although she received a lot of money,she ___ sad because she could never see her son again.
A.carry on （with sth/doing sth) 继续做。。。B.went on （with sth/doing sth) 继续做。。。C.remained 系动词，此处后接形容词sad做表语D.kept on（with sth/doing sth) 继续做...;继续行进。A,B,D的表达意思差不多，就高考而言，一般不去区分他们，所以答案是C
Some airline companies are now doing everyting they___ bring forward cost-saving ticketless flight.
A.can B.can to C.could D could to 此处关键词就是前面的are,说明了这是一般现在时态，所以用can最好，而如果把are 换成were,再去掉now的话，答案就可以选D了，所以这题答案是B。
The secretary has a lot of things to ___ in the offic since she has been away for quite a few days.
A,take up B. make up 当然不能选A了，因为take up是"占据时间，或空间"的意思，比如，e.g The old table takes too much room here,so we had better move it away. 而make up 在这里是“弥补”的意思，它还有“化妆，编造，组成”等意思，都是高考考点，所以答案是B

1,remain 表示依旧………… 而A,B,D三个选项都表示继续做什么，其后面的结构是carry/keep/go on doing sth 或直接加sth。

2，这里用can是最好的答案，第一，表示能力，第二，也与时态相照应。但could虽然可以表示能力，但是在时态上有点不相符。但是在一般情况下两者都是可以互换的。