You ran PCA on the daily returns of the S&P 500 constituents over the last year. What practical financial meaning do the first two principal components usually represent, and how would you use them in a statistical arbitrage strategy?

你对过去一年标普500所有成分股的日收益率跑了主成分分析（PCA）。前两个主成分通常代表什么实际的金融意义？你会怎么把它们用在统计套利（StatArb）里？