This is the 1257th most frequent Japanese word.
原因
Cause; reason.
In this question, '原因' signifies the primary factor or driving force contributing to the observed environmental issues.
環境問題の主な原因は何ですか？
What is the main cause of environmental problems?
The word '原因' refers to the cause or reason for an event or a situation, specifically the accident in this sentence.
事故の原因を調査しています。
We are investigating the cause of the accident.
Here, '原因' is used to describe the identifiable reason behind someone's failure, implying it is understood or easily determined.