努力是成功的关键。

Hard work is the ___key to success.

对于那些说钱买不到幸福的人，我回答说钱是幸福的一个关键。

To those who say that money can't buy happiness I answer that money is a ___key to happiness.

有效的规划是按时完成项目的关键。

Effective planning is the ___key to completing the project on time.