Which solution should you recommend?

You are the Office 365 Administrator for your company.
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?

A.
Implement SharePoint 2013 with Excel Services.

B.
Purchase an Office 365 Small Business subscription.

C.
Implement SharePoint Online and continue using Excel 2010.

D.
Purchase an Office 365 Enterprise El subscription.

Explanation:
By using Excel Services, you can reuse and share Excel workbooks on SharePoint 2013 portals and
dashboards.
https://msdn.microsoft.com/en-us/library/office/ms546696.aspx