A1-Possessive with apostrophe

There are two possibilities: 

1. The person who has something is singular
My sister has a cat. It is black 
My sister's cat is black  

2. The possessor is plural
The schools have classrooms. They are big. 
The schools´classrooms are big

Do the following online activities: 

• Activity 1
• Activity 2
• Activity 3

3. The question word WHOSE to ask about possession

Whose is this book? it is Bill's 

This question word, whose, is often confused with the contraction of who is = who's because the two forms are pronounced identically. 

Compare
Who's Peter? The man with the newspaper
Whose newspaper is this? it is Peter's 

• Activity 1