$2$ Which of these statements would need a citation because it refers to statements based on statistics?

a$.$ The main problem search engines have to deal with is the size and rate of change of the Web, with no search engine indexing more than one$-$third of the publicly available Web.

b$.$ Back in $2004,$ the average working day consisted of dealing with $46$ phone calls, $15$ internal memos, $19$ items of snail mail, and $22$ email messages.

c$.$ Search engines deal with a vast amount of material. The process of collecting and organizing that material becomes ever greater as the Web expands.