The most credible source in terms of researching the effects of smoking is letter b, research article in a scientific journal. It is because a credible source means that it is valid or could be relied on in which the research article is, because it has been researched and holds validity of which contains proof. Other choices does not correspond to the question for they don't hold any validity or proof in the topic that could be reliable.
The answer is A and C combined so final answer is D
Symbols are important because they facilitate communication and identification of ideas and other concepts based on what those symbols represent, though they can have literal as well as figurative meanings.Symbols can be used to signify individuals, groups of people, organizations or more ambiguous concepts
