It should be A. i believe
Answer:
<h3>A DOI, or Digital Object Identifier, is a string of numbers, letters and symbols used to permanently identify an article or document and link to it on the web. A DOI will help your reader easily locate a document from your citation.</h3>
If you mean polio then <span>it dropped by 6000 in 1962 or the first year when the vaccine was released</span>