The main concern of the passage is to _____ .

The unfavourable effects of cigarette smoking on the
heart have frequently been described, but the exact basis
for these effects has not been clarified. Some
investigators believe nicotine to be culprit and there has
been some experimental work in animals indicating that
large doses of nicotine in conjunction with cholesterol
feeding and vitamin D could produce a disease of the
arteries resembling that seen in humans. An alternative
explanation has been offered by other scientists who have
pointed to the possible role of carbon monoxide being
inhaled with the cigarette smoking.