Which solution should you recommend?

You plan to implement a collaboration platform for a company. All 1,000 employees currently use Excel 2010.
You have the following requirements:
-All employees must be able to co-author Excel workbooks.
-You must minimize the costs associated with any solution.
You need to recommend a solution.
Which solution should you recommend? can co-author documents in Word, PowerPoint, Excel, and OneNote Web Apps. However https://blogs.office.com/2013/11/19/real-time-co-authoring-in-the-excel-web-app-why-and-how-we-did-it/

A.
Implement SharePoint 2013 with Excel Services.

B.
Implement SharePoint 2013 and continue using Excel 2010.

C.
Implement SharePoint Online and upgrade clients to Excel 2013.

D.
Implement SharePoint 2013 with an Office Web Apps server.

Explanation:

Excel services consists of three components: Excel Calculation Services, Excel Web Access, and Excel Web
Services.
Excel Web Access allows for co-authoring of Excel workbooks.
Incorrect Answers:
B: To be able to co-author Excel workbooks we would need either Excel Web App or Office Web Apps.
C: Upgrading to Excel 2013 would not help. To be able to co-author Excel workbooks we would need either
Excel Web App or Office Web Apps.
D: Co-author can implemented through an Office Web Apps Server, but it would not minimize the cost.
If you are using SharePoint Online or have SharePoint 2013 configured to use Office Web Apps Server, users
References: