Question

 85. 父亲听到这个消息很兴奋。(excited)

86. 不经历风雨，怎能见彩虹。(without)

87. 众所周知，他的成功完全是努力的结果。(due to)

88. 当你怀疑自己的时候，不要忘了最重要的是建立自信。(doubt)

89. 为了按时完成任务，志愿者们正竭尽全力把复杂的问题简单化。(simplify)

Answer 1

85. Father was very excited to hear the news/when he heard the news.

at the news/(on) hearing the news 

86. There is no rainbow without wind and rain.

   How can you see the rainbow without going through the storm?

87. It is known to all that his success is entirely/totally/completely due to his hard work.

       /As is known to all, his success is entirely/totally/completely due to his hard work.

       /We all know that his success is entirely/totally/completely due to his hard work.

88. When you doubt yourself, don’t forget that it is the most important (thing) to build up self-confidence.

89. In order to accomplish/finish/complete the task in time, the volunteers are trying their best/doing all that/what they can to simplify the difficult/complicated matters.