用issue,problem和question填空

题目

用issue,problem和question填空
There have been so many trade______ since China joined the WTO.
The best way is to get to the root (heart) of the_____.
Unemployment is a very serious ______for graduates now.
The key ______of what caused the leak remains unanswered.
In an interview try to ask some open _____ that don't just need "Yes" or "No" as answers.

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