It's been a tough week for the bulls. The selling in the stock market continued on Friday, extending a decline that puts major indexes on track for a sharp weekly loss. The Nasdaq was down more than 4% in five days, while the S&P 500 lost more than 2%.Here's where US indexes stood shortly after the 9:30 a.m. opening bell on Friday: There are a few takeaways emerging on Wall Street — and one level that forecasters say they're watching. Here's what you need to know: 1. It's all about valuations…The sell-of ...