 The U.S.-listed shares of Petroleo Brasileiro SA PBR, -3.34 PETR3, -2.38 sank 3.3 in very active afternoon trading Tuesday, putting them in track for the first sub-10 close in exactly a year, after UBS swung to bearish on the Brazil-based oil and gas giant, citing concerns over three transformational points: fuel prices, investments and overhead. Trading volume swelled to 49.1 million shares, making the stock the most actively traded on the New York Stock Exchange NYSE. 