This paragraph is a little long. But, I really want a help. I am confusing which one is the answer. And thank you.

*Read th article and answer the question

Gold's beauty, usefulness and rarity have given it a value unsurpassed for thousands of years. In 334 B.C., Alexander the Great marched 35,000 men into Persia to seize storages of the glittering metal. The gold that Julius Caesar grabbed during his conquest of Gaul enabled him to change the face of Europe.

In 1492, Christopher Columbus's eye caught sight of gold on the island of hispaniola. Over the century that followed, Spanish conquistadors sent back from the new World vast amounts of the precious metal. The circulation of gold coins all over Europe leapt dramatically.

Gold quickly became a universal currency. English merchants in the 17th century, needing to store more gold than they could securely hide themselves, began leaving it with goldsmiths in return for a receipt. Trading receipts soon became easier than lugging around sacks of coins. Eventually the goldsmiths, realizing receipts were as good as gold, wrote their own and traded them for goods, popularizing the concept of a ( )

Q) What is the proper word for the blank above?

a. gold

b. coin

c. receipt

d. check

e. paper money.

I know I have to give my answer first, but, if I give the answer(my own or given in my book), it might affect your answer. Therefore, I will write the answer after sombody's solution.

My answer was two. But, according to a book, there was only one answer. That's why I wrote this question in this page. Hope sombody to read this long article.

Because of the word "a", the answer has to be a countable noun. This rules out gold and paper money.
"receipt:" is used several times previously, so "a receipt" must be ruled out on the grounds that a concept of a receipt would not be defined in terms of itself. Further, a receipt is not made of metal, like a coin, so "a coin" cannot be correct.

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Thank you.

My answer was 'check' also. But the anwer given in the book was 'paper money'.

Except me, others all agreed to the anwer 'paper money'.

Somebody come to Korea and please tell others the answer is 'check' or both ('check' or 'paper money')
"concept of a paper money" just doesn't seem right.
"concept of paper money" would be all right, though.

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