Optional argument
[计] 任选变元
🧩 词语组成解析
🗣️ 情景例句
It's your birthday.
今天是你的生日。
One usually pricks up one's ears when his name is called.
人在被叫到自己的名字时，通常要侧耳倾听。
A cat's ability to survive falls is based on fact.
猫在跌落时能够大难不死是有事实作为依据的。