SEOUL, South Korea (AP) — With leader Kim Jong Un in attendance, North Korea has opened a key political conference to discuss improving its struggling economy and reviewing defense strategies in the face of growing tensions with rivals. That’s according to state media reports Saturday. The enlarged plenary meeting of the ruling Workers’ Party’s Central Committee came as the United States sent a nuclear-powered submarine to South Korea. It’s the allies’ latest show of force against the North, which has ramped up its testing of nuclear-capable missiles to a record pace in recent months.