Wall Street couldn't extend a record-breaking streak into a third session following the weekend terror attack in London. Crude oil also dropped to begin the week. Apple  (AAPL - Get Report) held lower after kicking off its Worldwide Developers Conference. The world's largest company unveiled a new Apple Watch face powered by Siri and a new iMac Pro model, its most powerful yet. Apple also announced that Amazon Prime video would be added to Apple TV this year. WebMd  (WBMD) rose on reports IAC  (IAC - Get Report)  was interested in an acquisition. IAC is reportedly considering purchasing the medical search engine for a roughly 7% premium to its trading levels on Friday. Mallinckrodt (MNK - Get Report)  was one of the worst performers on the S&P 500 following a report from short-seller Citron Research. Citron accuses the company of working with Express Scripts  (ESRX) to sell one of its hormone gels for a large mark-up.