CNBC.com's Jordan Smith brings you the day's top business news headlines. On today's show, CNBC Digital reports on comments made by SEC chair Gary Gensler that threaten Robinhood's biggest source of revenue. Plus, the United States ends its longest war after nearly 20 years.

Robinhood tanks after SEC chair tells Barron’s that banning payment for order flow is a possibility

Shares of Robinhood dropped Monday amid several bouts of bad news for the brokerage app.

Robinhood's stock fell 6.9% to $43.64 per share after Securities and Exchange Commission Chairman Gary Gensler told Barron's that banning the controversial practice of payment for order flow is "on the table."

Gensler told the outlet that payment for order flow — the back-end payment brokerages receive for directing clients' trades to market makers — has "an inherent conflict of interest."

Payment for order flow is one of Robinhood's largest revenue sources and the way the millennial-favored stock trading app is able to provide zero-commission trading. Payment for order flow is a controversial practice that has garnered attention from the Financial Industry Regulatory Authority and Main Street.

Asked for clarification, an SEC spokesperson declined to comment for this story. Gensler has for months said that an outright ban of payment for order flow is among several options the regulator could introduce.

China to ban kids from playing online games for more than three hours per week

Kids and teens under 18 years old in China will only be allowed up to three hours per week to play online video games, according to new rules published Monday by China's National Press and Publication Administration.

The move is a fresh blow to the country's gaming giants from Tencent to NetEase, which have dealt with an onslaught of regulation this year in areas from anti-monopoly to data protection. That has spooked investors and dented the value of Chinese tech stocks.

According to a translated notice about the new rules, people under 18 will be allowed to play video games one hour a day between 8 p.m. and 9 p.m. weekends and legal holidays. The agency billed the rules as a way to safeguard children's physical and mental health.

Biden says the era of U.S. nation building is over as he marks the end of the Afghanistan war

President Joe Biden gave a full-throated defense of his decision to end the U.S. war in Afghanistan after 20 years of conflict, saying the era of large U.S. military deployments to rebuild other nations has ended.

Biden's address Tuesday came just 11 days before the 20th anniversary of the Sept. 11, 2001 terrorist attacks that precipitated the U.S intervention in Afghanistan.

"My fellow Americans, the war in Afghanistan is now over," Biden said from the White House. "I'm the fourth president who has faced the issue of whether and when to end this war."

"When I was running for president I made a commitment that I would end this war and today I have honored that commitment. It was time to be honest with the American people, we no longer had a clear purpose in an open-ended mission in Afghanistan," he added.