Monday saw a strong session for the stock market, as the Dow Jones Industrial Average posted triple-digit gains and other major benchmarks were higher by about 1%. Most investors celebrated news from the G-20 summit in Argentina, at which Presidents Xi and Trump agreed to a calming of trade tensions to allow for a 90-day period of heightened negotiations. Many stocks climbed in anticipation that reduced tariffs could benefit both countries, and some good news on the mergers and acquisitions front also lifted shares of certain companies. Cronos Group (CRON -3.25%)Tesaro (NASDAQ: TSRO), and Wynn Resorts (WYNN -1.87%) were among the best performers on the day. Here's why they did so well.

Has Cronos gotten Altria's attention?

Cronos Group shares finished higher by 11% after reports surfaced that the Canadian marijuana specialist could be in merger talks with tobacco giant Altria Group (MO -1.00%). Many have speculated that Altria could look at a closer collaboration with a cannabis company in order to tap into the potentially lucrative market, but it was interesting to see the Marlboro maker contemplate a full acquisition. Most had expected Altria to follow in the footsteps of Constellation Brands, which took a sizable minority stake in Cronos' peer Canopy Growth. Others had thought Altria might go with a competitor rather than Cronos. But the deeper move comes as Altria is already reportedly considering taking a stake in e-cigarette specialist Juul Labs, showing that the cigarette giant is looking for ways to grow as its primary market keeps declining.

Interior view of a commercial marijuana growhouse

Image source: Getty Images.

Tesaro's never been healthier

Shares of Tesaro soared 58.5% in the wake of the drugmaker getting a buyout bid from industry giant GlaxoSmithKline. Under the terms of the agreement, Glaxo agreed to pay $4.16 billion for Tesaro, giving investors $75 per share in cash for every Tesaro share they own. For Glaxo, the move would potentially expand its pipeline, with key Tesaro treatments including its Zejula drug for ovarian cancer. For Tesaro, the acquisition halts a year-long slide in its shares and could be a reasonable exit point for longtime shareholders.

Wynn wins a big bet

Finally, Wynn Resorts stock finished higher by 9.5%. The casino resort giant got a boost from the latest report on the state of the industry in Macau, where gambling revenue rose by 8.5% in November compared to year-earlier levels. Throughout much of the year, Wynn has struggled to overcome the challenges raised by allegations that led to the departure of former CEO Steve Wynn, and trade tensions between the U.S. and China led to concerns about the health of the Chinese economy. Yet after some struggles, Macau has started to rebound, and that's good news for Wynn and the whole industry going forward.