The need to communicate is a key characteristic of human society.

　　需要交流是人類社會最重要的一個特征。

　　Cost was a key element in our decision.

　　價錢是我們決策時考慮的主要因素。

　　Good communication is key to our success.

　　良好的溝通是我們成功的關鍵。

　　These units played a key functional role in the military operation.

　　這些單位在軍事行動中起到了主要的職能作用。

　　The president appears likely to change course on some key issues.

　　總統看起來可能要在某些重要問題上改變方針。