Based on the passage provided, here are the best answers for each question:

**26. Which sentence is NOT true?**
- **c. The Green Revolution caused the population explosion in the 20th century.**
(This is not true because the Green Revolution increased agricultural productivity, which contributed to population growth, but it did not directly cause the explosion.)

**27. When is an area considered “under-populated”?**
- **a. When its population is not big enough to maintain the economic system.**
(This aligns with the definition provided in the passage.)

**28. Which areas are expected to encounter a decrease in population growth in the next decades?**
- **d. B and C (Some countries in Western Europe, and Japan).**
(The passage indicates that both Japan and some countries in Western Europe are expected to face decreased population growth.)

**29. Negative population growth in Central and Eastern Europe is due to…**
- **a. high fertility rate.**
(This would not be correct; the passage attributes negative growth to low fertility rates.)

The correct answer for question 29 is likely **c. low death rate** as low death rates would not lead to negative population growth; however, the passage indicates low fertility rates are causing the negative growth.