Answer:
The correct answer is: D - There are numerous examples of the ways scientists work to make people's lives better.
Have a great day! :D
When could is used it refers to an ability that <span>a person generally had in the past or to something that was generally possible in the past, can is present.
I could wash the dishes for you. (having the ability to do it)
I can wash the dishes for you. (its actually going to be done)
hope this helps you! :-)</span>
It's known as abstract nouns