What is the relationship between 'South32' and 'nickel' in the context of the input sentence.
Options: industry, owned by, product/material produced, distribution format

But Citi also said there was potential South32 would run the ruler over long-life base metal assets that would fill the company's growth "void" and offset the relatively short life of its Cannington silver, lead and zine mine in Queensland and declining grades at the Cerro Matoso nickel mine in Colombia.
product/material produced