BEIJING — China said on Thursday that it would temporarily exempt foreign companies from paying tax on their earnings, a bid to keep American businesses from taking their profits out of China following Washington’s overhaul of the United States tax code. There is, however, a catch: To be eligible, foreign companies must invest those earnings in sectors encouraged by China’s government — including railways, mining, technology and agriculture — according to a statement from the Finance Ministry. The measure is retroactive from Jan. 1 this year, the ministry said. The move would “promote the growth of foreign investment, improve the...