Both, A and R are correct and R is the correct explanation of A.
The geographical extent of Kushana Empire ensured that an intimate contact was established between India and Central Asia. As a result, India received a good deal of Gold from the Altai Mountains of Central Asia. Gold may also have been received through the trade with Roman Empire. Kushanas controlled the Silk route that stretched from China via Kushan controlled Central Asian territory up to the Mediterranean World. The toll levied from this flourishing trade enriched Kushanas. Thus, they could issue large number of Gold coins to pay for the high value transactions.
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