BP AND SHELL, two British oil giants, have long sunk cash into solar and wind farms. Their rivals elsewhere have mostly stuck to their drilling. Investors have rewarded single-mindedness. ExxonMobil, an American firm unapologetically wedded to the black stuff, is worth $510bn, half as much again as the British duo combined. Its share price is up by 50% in the past five years, compared with a rise of 10% for Shell and a fall of 13% for BP.