根据句意和汉语注释,写出单词的正确形式。
The children are too young to sit __________(从头到尾) a concert(音乐会).
_________(物理) is a little difficult, but it is very useful.
At first, we spent much time ___________(讨论) the project with the famous engineer.
Do you want to go to any __________ (外国的) countries in the future?