U.S. stocks fell at the open on Thursday to put the S&P 500 on track for a fourth straight decline, after Saudi Arabia and its allies launched air strikes on Yemen. Oil prices spiked, with Brent crude up 2.4 percent to $57.86 and U.S. crude up 3 percent to $50.69 after warplanes from Saudi Arabia and its allies struck Shi’ite Muslim rebels fighting to oust Yemen’s president. “Obviously the situation in Yemen is being used as the excuse for this pullback, which is a continuation of yesterday,” said Peter Cardillo, chief market economist at Rockwell Global Capital in New York. U.S.