Bitcoin’s price plummeted after U.S. regulators rejected a proposal by the Winklevoss twins for a publicly traded fund based on the digital currency, dashing hopes that a government-approved investment vehicle would lead to wider interest in virtual money. The Securities and Exchange Commission refused to grant an exemption that would have let the Winklevoss Bitcoin Trust trade on the Bats BZX Exchange, according to a filing posted Friday on the regulator’s website.