Here are the completed sentences:

1. Let's go to the zoo this Sunday.
**Why?** → **Because it's fun.**

2. The school is in front of the bookstore.
**The bookstore is** → **behind the school.**

3. It is not good to play cards all night at Tet.
**You shouldn't** → **play cards all night at Tet.**

4. What is your address?
**Where** → **do you live?**

5. A motorbike is often cheaper than a car.
**A car is often** → **more expensive than a motorbike.**