<span>determining the general purpose
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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.
A bigwig refers to a person who is an expert in a particular area of study or field. Thus, a bacterial wig will be someone who is an expert in the area of bacteriology. Such a person will probably have the highest degree that is obtainable in that field and will be an authority to reckon with when it comes to the issue of bacteria.
