【单选题】

If open-source software is supposed to be free, how does anyone selling it make any money It’s not that different from how other software companies make money.

You’d think that a software company would make most of its money from, well, selling software.But you’d be wrong. For one thing, companies don’t sell software, strictly speaking; they license it. The profit margin on a software license is nearly 100 percent, which is why Microsoft gushes billions of dollars every quarter.
But what’s the value of a license to a customerA、license doesn’t deliver the code, provide the utilities to get a piece of software running, or answer the phone when something inevitably goes wrong. The value of software, in short, doesn’t lie in the software alone. The value is in making sure the soft ware does its joB、Just as a traveler should look at the overall price of a vacation package instead of obsessing over the price of the plane ticket or hotel room, a smart tech buyer won’t focus on how much the license costs and ignore the support contract or the maintenance agreement.
Open-source is not that different. If you want the software to work, you have to pay to ensure it will work. The open-source companies have refined the software model by selling subscriptions. They roll together support and maintenance and charge an annual fee, which is a healthy model, though not quite as wonderful as Microsoft’s money-raking one. Tellingly, even Microsoft is casting an envious eye at aspects of the open-source business model. The company has been taking halting steps toward a similar subscription scheme for its software sales. Microsoft’s subscription program, known as Soft wareAssurance, provides maintenance and support together with a software license. It lets you up grade to Microsoft’s next version of the software for a predictable sum.But it also contains an implicit threat: If you don’t switch to SoftwareAssurance now, who knows how much Microsoft will charge you when you decide to upgrade
Chief information officers hate this kind of "assurance", since they’re often perfectly happy running older versions of software that are proven and stable. Microsoft, on the other hand, rakes in the software-licensing fees only when customers upgrade. SoftwareAssurance is Microsoft’s attempt to get those same licensing fees but wrap them together with the service and support needed to keep systems running. That’s why Microsoft finds the open-source model so threatening: open-source companies have no vested interest in getting more licensing fees and don’t have to pad their service contracts with that extra cost. In the end, the main difference between open-source and proprietary software companies may be the size of the check you have to write.
"That extra cost"(Line 7, ParA、5) probably refers to
A、software-licensing fees.
B、total cost of a software product.
C、payment for service contracts.
D、the charge for subscription.