Answer: The correct answer for the blank is -
must be mined from underground.
Oil is considered as a fossil fuel as it is formed by the dead remains of plants and animals that lived on earth millions of years back.
It is a non renewable source of energy because it can not be replenished readily by natural means and to renew, it would again require millions of years.
Mining of oil is a process of extraction of fossil fuels from underground, which is based on the digging of underground mining excavations.
Thus, oil must be mined from underground is the right answer.
