U.S. closed sharply lower Thursday amid mixed economic data, while retail stocks fell on disappointing earnings. Walmart and Kohl's both failed to meet revenue expectations. Ten year Treasury yields touched their lowest level in almost a year, triggered by fresh speculation of more European stimulus after the eurozone economy grew just 0.2% in the first quarter. David Tepper, founder of $20 billion hedge-fund firm Appaloosa Management described markets as "dangerous" and said he was worried the U.S. economy wasn't growing fast enough. Russian President Vladimir Putin has said the country will only deliver gas to Ukraine if it pays in advance in cash, starting from June. Jane Searle reports from Wall Street.