The United States was attempting to force china to trade with them regardless if they wanted too or not.
Metallurgy was well known by the Guptas,.as can be attested to by the iron pillar of Delhi. The correct option among all the options that are given in the question is the third option. This pillar is a glowing example of the metallurgical advancement that the Guptas had during that time. This pillar has not rusted for a long time.
