Answer:
a. natural environment.
Explanation:
Many of the technological advances, most notably the ones that came about during the 20th century, that have made human life possible at such a great scale nowadays have also produced environmental problems that threaten the very existence, not only of humans on Earth, but of all life on the planet. The clearest example of an environmental problem today is global warming which many scientists argue has reached a phase of no return, which means that its consequences will be felt all around the world and there is nothing that can be done to stop them (which does not mean we should not reconsider our lifestyles and our systems of production).
