III. Choose the word that best fits the blank space in the following passage. (1.5 pts

Renewable resources
When it comes to energy resources, there is always the (17) __________ of sustainability. It is important that resources provide enough energy to (18) __________ our needs-to heat our houses, power our cities, and run our cars. However, it is also important to consider how these resources can be used long term. Some resources will practically never (19) __________out. These are known as renewable resources. Renewable resources also produce clean energy, meaning less pollution and greenhouse gas emissions, which contribute to climate change.
There are some challenges associated with using renewable resources. For instance, renewable energy can be (20) __________ reliable than non-renewable energy, with (21) __________ or even daily changes in the amount produced. However, scientists are continually (22) _________ these challenges, working to improve feasibility and reliability of renewable resources.
http://www.nationalgeography.org/encyclopedia/renewable-resources/