Bitcoin and other popular cryptocurrencies plummeted on Friday after the Chinese government declared all cryptocurrency activities (including overseas transactions) illegal and vowed to clamp down on businesses related to digital currencies. In a public notice posted on Friday local time, China’s central bank, the People’s Bank of China (PBOC), said businesses that offer trading, token issuance and derivatives for cryptocurrencies are all prohibited. Overseas crypto exchanges providing services in mainland China are also illegal, according to the notice. The PBOC, as well as China’s internet and securities regulators, will develop “new systems” to monitor and reduce risks posed by cryptocurrencies....