“We've got to find a viable alternative to fossil fuels.”
You're discussing social issues with a couple of friends. One issue that's important to you is the environment. You think it's important for your country to develop "green" energy, so you say this.
We've got to find a viable alternative to fossil fuels.
"Got to" means "must". You can use this phrase to talk about something that you think is important for a group of people that you're included in. For example:
We've got to communicate with each other better.
An "alternative" is a choice or a possible solution to a problem.
A "viable alternative" is a choice or possible solution which might work.
To illustrate this with an example, imagine that you need to get to work but you don't want to drive a car. Walking to work is not a "viable alternative" because you live too far away from your office. But maybe riding a bicycle is a "viable alternative".
Alternatives can be "viable" or not "viable" in a few different ways:
- economically viable
- politically viable
- commercially viable (able to be sold)
"Fossil fuels" are fuels like oil (which is used to make gasoline), natural gas (which is used to heat homes and for cooking), and coal.
These fuels are called "fossil fuels" because they come from decayed plants and animals from a very long time ago. A "fossil" is the remains of a plant or animal which died a very long time ago.