Based on the information provided, here are the most likely answers to the questions related to the online lecture about living underground:

1. **Why is there a need for a new solution to the problem of where to live?**
- **c. The world’s population is growing very quickly.**
This answer suggests that the increasing population necessitates finding new living spaces, underscoring the urgency for innovative housing solutions.

2. **Why are architects creating plans for underground buildings in cities?**
- **b. There is little room left on the surface.**
This answer indicates that the lack of available space on the surface of cities is a primary reason for architects to explore underground living options.

Make sure to listen carefully to the lecture for confirmation of these answers!