根据句意及汉语提示，写出各单词的正确形式，每空一词。
Perhaps no one has known the     (原因) of this kind of disease so far.
Alice is full of energy and does not mind doing       (额外的) work for you.
Our English teacher gives us a great deal of      (建议) on how to remember words.
President Xi said the      (友谊) between China and the UK would have a bright future.
It’s reported that the teachers of Hengshui High School are     (严格的) in English handwriting than in many other schools.