After a two-day plunge, the stock market mostly advanced, suggesting that perhaps Wall Street will be successfully weaned from the Fed’s easy money. ‘‘Saner heads are prevailing,’’ said one investment chief. “The economy is getting better.’’ The Fed’s bond buyback program had been a big driver behind the market’s four-year bull run. Makers of consumer staples, utilities, and health care companies rose the most. Only tech stocks fell. Darden Restaurants, which runs Olive Garden and Red Lobster, fell 2 percent on rising expenses that hurt earnings. And Facebook rose after saying it will add video to its photo-sharing app Instagram.