key 
英 [kiː]  美 [ki] 
n. 钥匙；线索；按键；调；关键

v. 为…调音；用键盘输入

adj. 最重要的；关键的

例句：
He locked the bag and put the key in his pocket. 

他锁好了包并把钥匙放进了口袋。

Where are my car keys? 

我的车钥匙在哪里？

a key player in my team 

我队的关键队员

Marketing is key to getting a product recognized in the market. 

市场营销是使产品被市场认可的关键。

Airports are key in attracting overseas investment to the UK. 

机场是吸引海外对英投资的关键。