1. Pig - racing/ often/ hold/ country fair/ entertainment/.

→ Pig racing is often held at a country fair for entertainment

2. Noise pollution/ cause/ change/ animals’/ living pattern.

→Noise pollution causes a change in animals' living pattern

3. What/ happen/ if/ there/ be/ no/ car/ world?/

→ What would happen if there were no cars in the world?

4. London/ have/ population/ eight million people.

→London has (have) a population of eight million people
Ex 5:
17.B. have / would buy 

18. B. were /would