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Netflix surged 16.5 percent, the biggest gain on the S&P 500. After trading ended Wednesday, the streaming video company reported that its fourth-quarter earnings had climbed sixfold and that it had added 2.3 million subscribers during the period. Tech stocks fell less than the rest of the market on Thursday. Netflix predicts 2.25 million new domestic subscribers this quarter. It estimates first-quarter profit of $48 million, or 78 cents a share, versus analysts’ forecasts of 75 cents.