Who or what is it?

It's a person who sells flowers.

(a)  

It's a shop that sells food.

(a)  

It's a shop that sells shoes to people.

(a)  

It's a thing which you use to call someone.

(a)  

This is the season that is very hot.

(a)  

This is a school which students live in.

(a)  

It's a building which collects parcels and letters.

(a)  

It's a place which some people visit every Sunday.

(a)  

It's a thing which you can see at a flea market.

(a)  

It's a polite phrase which you use when you need help.

(a)