Warren Buffett offered a vigorous defense of Goldman Sachs Group Inc. Saturday, saying the embattled firm hadn't engaged in improper activity and shouldn't be blamed for the losses of its clients. Goldman has been reeling from Securities and Exchange Commission allegations that the bank had engaged in fraudulent activities in relation to a mortgage deal called Abacus 2007-AC1. Goldman says it did nothing wrong. The billionaire investor said he fully supported Goldman CEO Lloyd Blankfein. Asked if he could choose a successor for Mr. Blankfein, Mr. Buffett said: "If Lloyd had a twin brother I'd go for him." Buffett also...