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Open enrollment in Obamacare starts Sunday, in what appears to be a pivotal time for the controversial healthcare law.

Obamacare started on a rocky road in 2013 with the botched rollout of healthcare.gov. The website has since improved and added new features, and about 10 million people have paid for Obamacare on the insurance exchanges.

But challenges remain for the marketplaces to remain viable, a key indicator for the healthcare law. Here are four key challenges facing them:

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