we are thinking about ------we will travel to Taiwan next we

选C
翻译为：我们正在考虑下周是否去台湾旅游
解析：if与whether都是“是否”的意思,在此句中,we will travel to Taiwan next week 是表语从句,所以应用whether
具体if与whether的区别如下：
if和whether都可译为\"是否\",二者引导宾语从句时通常可以互相替换,口语中多用if代替whether.例如：Ask if/whether he plans to come to the meeting.而以下几种情况中,只能用whether:
1.强调两方面的选择,特别是句中有or not时.如：
Let me know whether you can come or not.
2.宾语从句前置时.如：
Whether this is true,I can\'t say.
3.引导主语、表语、同位语从句时.如：
Whether the football game will be played depends on the weather.（主语从句）
The question is whether he can do it.（表语从句）
The question whether we should go with them must be decided at once.（同位语从句）
4.在介词之后作介词的宾语时.如：
I haven\'t settled the question of whether I\'ll go back home.
5.在不定式前与不定式一起组成词组时.如：
Whether to go or stay is still a question.
6.在discuss,decide 等动词后作宾语时.如：
We discussed whether we should go there by plane.