The software giant’s stock closed down 10%, its biggest plunge since March 2020, following Microsoft’s earnings after the bell Wednesday, which showed record spending on artificial intelligence as growth at its key cloud unit slowed. The only bigger one-day valuation destruction was Nvidia Corp.’s $593 billion rout last year after the launch of DeepSeek’s low-cost AI model. Microsoft’s move is larger than the market capitalizations of more than 90% of S&P 500 Index members, according to data compiled by Bloomberg.