What group of protists is thought to be the main source of earth's oil deposits?

The answer is; plankton

The major group of plankton that contribute to earth’ crude oil are phytoplanktons because of their  great numbers – even though they are microscopic. They are the base of the food chains in the oceans because they are the main primary producers. Examples of phytoplankton are the dinoflagellates, cyanobacteria,  diatoms and blue-green algae.

answer; /// during daylight hours the sun heats up the ground,and at night the clouds insulate the warmth making it warmer ;

it sounds like either a or c