The correct answer is C. Trading was very popular at that time (during the Warring States period), in fact, a lot of money and goods were sold which allowed to create a larger merchant class.
Coins were used by the first time for Lydians (people from the ancient kingdom of Lydia) in the seventh century BC, so it is not possible the option A. After that century, some other cultures started to trade using different coinage and with different shapes, including the Zhou dynasty.
In case of the option B, actually the dynasty Zhou ended up because of the Warring States period in 256BC, so it is not possible as well.
And finally the Gobi Desert was used for isolating China from the rest of the world. The only ones who lived there were the Mongols.
