Fill in the blanks:
We must explore ___________ sources of energy as our supply of fossil fuel has been___________.

Correct option is A

The correct answer is (a) alternative; depleted.
· Alternative sources of energy: Refers to renewable or substitute energy sources like solar or wind energy that are considered when conventional energy sources (fossil fuels) are insufficient or depleted.
· Depleted: Refers to the gradual reduction or exhaustion of resources, such as fossil fuels.
Why the other options are incorrect:
· Guaranteed; over:
· "Guaranteed sources" does not fit the context, as energy sources need to be explored, not guaranteed.
· "Over" is too informal and lacks precision.
· Sufficient; increased:
· "Sufficient sources" contradicts the need to explore new energy sources, and "increased" does not describe the reduction of fossil fuel reserves.
· Natural; exhausted:
· While "exhausted" fits, "natural sources" is too vague and not as specific as "alternative sources."