$(3)$ Which of these statements should have citations because it refers to a research study or its conclusions?

a$.$ We ran a crawl of more than $100$ million Web pages on about $5$ million hosts. The crawl was performed over a period of $18$ days; however, the crawler was not continuously in operation during this time. Of the $138$ million pages cataloged, more than $400,000$ produced a $403$ error indicating that access was forbidden.

b$.$ Given the current size of the Web, even large search engines cover only a portion of the publicly available part. A report from the Websonite Group indicates that large$-$scale search engines index no more than $40\%$ to $70\%$ of the indexable Web.

c$.$ You can study the Web all you want, but there will still be more to learn.