Apple Inc.’s shares surged past the $500-threshold, their highest level since January.Wednesday’s gains came a day after activist investor Carl Icahn said he thought the iPhone maker should do more to revive its stock price. The outspoken billionaire said in Twitter posts on Wednesday that he had acquired an unspecified stake in Apple, and had spoken to its CEO about boosting share repurchase plans. The stock immediately jumped, and closed at its highest level in nearly seven months.