Read the following paragraph from the section "sleep cleans the slate."

another possibility is that sleep may clean out toxic proteins that are bad for the brain. during sleep, the space between brain cells increases. this allows harmful waste to be flushed out. in 2013, researchers published a study showing that the brain uses sleep to get rid of toxic proteins. this waste removal system could be one of the major reasons why we sleep. by removing these toxic proteins from the brain, sleep may keep people from getting brain diseases.

how does this paragraph contribute to the article's main idea?

a: it proves that toxic proteins are more present in the brain at night.

b: it explains exactly how harmful waste is flushed from the body.

c: it provides one possible answer to the question of why people sleep.

d: it describes the reasons why rem sleep is more important than nrem sleep.