The universal symbol of the Internet era communications...

Câu hỏi: Read the following passage and mark the letter A, B, C, or D to indicate the correct word or phrase that best fits each the numbered blanks.
The universal symbol of the Internet era communications, the @ sign used in e-mail addresses to signify the word 'at', is (26)_____a 500-year-old invention of Italian merchants, a Rome academic has revealed. Giorgio Stabile, a science professor at La Sapienza University, claims to have stumbled on the earliest known example of the symbol's use, as a(n) (27)_________of a measure of weight or volume. He says the sign represents an amphora, a measure of capacity based on the terracotta jars used to transport grain and liquid in the ancient Mediterranean world.
The professor unearthed toe ancient symbol in the course of research for a visual history of the 20th century, to be published by the Treccani Encyclopedia. The first (28) ___instance of its use, he says, occurred in a letter written by a Florentine merchant on May 4, 1536. He says the sign made its way along trade routes to northern Europe, where it came to represent 'at the price of', its contemporary accountancy meaning.
Professor Stabile believes that Italian banks may possess even earlier documents (29)_________the symbol lying forgotten in their archives. The oldest example could be of great value. It could be used (30) __________ publicity purposes and to enhance the prestige of the institution that owned it, he says. The race is on between the mercantile world and the banking world to see who has the oldest documentation of @.
The word or phrase which best fits the gap (27) is...
A. proof
B. sign
C. evidence
D. indication