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A trader works on the floor of the New York Stock Exchange.

Check out which companies are making headlines before the bell:

ConAgra — ConAgra and activist investor JANA Partners amended their existing cooperation agreement, with the foodmaker's board agreeing to nominate JANA designees Bradley Alford and Timothy McLevish as directors. The agreement also retains various standstill provisions for JANA. Separately, a Barron's article said ConAgra shares could rise up to 30 percent, as the food company sells businesses, improves existing brands, and lowers expenses.

Westar Energy — Westar, the largest electric utility in Kansas, is being bought by Great Plains Energy , the parent of Kansas City Power & Light. The deal is worth $8.6 billion in cash and stock, or about $60 per share, about 13.4 percent above Friday's close.

Verizon — Details of the tentative deal between the telecom giant and its unions have been released, with the pact giving pay raises of more than 10 percent and includes 1,400 new jobs.

Starbucks — The coffee chain announced a rollout of a new portfolio of cold coffee beverages, which will become the foundation for a new "cold bar" at Starbucks stores.

Medtronic — The medical products maker reported adjusted quarterly profit of $1.27 per share for its latest quarter, 1 cent a share above estimates. Revenue also came in above forecasts. However, Medtronic does see fiscal 2017 profit of $4.60 to $4.70 per share, largely below the consensus estimate of $4.70 a share.

Micron Technology — Baird upgraded the chip maker's stock to "outperform" from "neutral," based on improving profit margins, a better cost structure, and price stabilization in the memory chip market, among other factors.

Viacom — Viacom independent directors say they will fight any attempt to remove them from the media company's board of directors, should controlling shareholder Sumner Redstone attempt to oust them. The board members wrote a letter to shareholders, also saying the assertion that Redstone is mentally competent is "inexplicable."

TransDigm Group — TransDigm will join the S&P 500 after the close of trading on Thursday. It will replace drug maker Baxalta , which is being acquired by Shire . TransDigm is a maker of aircraft components.

Volkswagen — Volkswagen surprised analysts by reporting an increase in first-quarter profit. The automaker earned $3.8 billion for the quarter, up slightly from a year earlier. Consensus forecasts had called for a 17 percent drop in the wake of the company's diesel emissions scandal.

Mondelez — Mondelez is developing more mobile apps based on its products following the success of an Oreo cookie-themed game. That game has been downloaded approximately seven million times since it was introduced 3½ years ago.

Alibaba — The China-based e-commerce company told vendors to stop selling drugs on its Tmall website because of government rule changes in China.

Oracle — Oracle and Hewlett Packard Enterprise head to court today in a dispute over Oracle's alleged role in the decline of Hewlett-Packard's hardware business prior to the spinoff of Hewlett Packard Enterprise and HP Inc.

Apple — Apple may extend its iPhone model update schedule to every three years, according to an article in Japan's Nikkei newspaper.

Celator Pharmaceuticals — Celator will be bought by Jazz Pharmaceuticals for $30.25 per share in cash, or about $1.5 billion. That represents a nearly 73-percent premium over Friday's close for Celator shareholders.

Monsanto — The agricultural chemicals and seeds maker may get an improved buyout bid from Germany's Bayer as soon as this week, according to the Sunday Times. Monsanto rejected Bayer's $122 per share offer last week.

Williams Cos. — Williams has told Energy Transfer Equity that it is open to considering a new bid from its rival pipeline company, in an effort to end a legal dispute between the two over terms of their previously struck $20 billion takeover deal.