WASHINGTON (MarketWatch) — The output of the nation’s factories, mines and utilities surged 0.9% in July as auto assemblies rebounded and consumers turned on their air conditioners, the Federal Reserve said Tuesday. The gain, the biggest since last December, was in line with economists’ expectations. The key question is whether the increase can be sustained in future months. Output in June was revised up to a 0.4% gain from the prior estimate of a 0.2% gain and output in May was also revised higher.