Think about a petal plucked from its flower. is the petal alive according to the characteristics of life ? why or why not ?

No.

It contains living cells (temporarily), but neither the cells nor the petal fit the definition of an organism: "any complete and entire living thing capable of carrying out all the processes necessary for life."

Technically, the petal as a whole is dead, but the cells within it are more or less still alive for the meantime as they don't die immediately after being plucked. It's like how if you shoot someone in the heart, they're still alive for a moment to a few minutes.

air enters through the nose, moves through the nasal cavity, the pharynx, the larynx, enters the trachea, moves through the bronchi andbronchioles till the alveoli.