[Practice Exam] Why is the answer?

Question

You are an administrator for several organizations in the same company. Each organization has data in their own BigQuery table within a single project. For application access reasons, all of the tables must remain in the same project. You think each organization should be able to view and run queries against their own data without exposing the data of organizations to unauthorized viewers. What should you recommend?

why answer is C?

Place the tables in a single dataset, and apply IAM roles to each table, limiting access per table to each organization.

why not D?

Create a separate dataset for each organization in the same project. Place each organization's table in each dataset. Restrict access to the organization's dataset to only that company, from which they can view their table but no one else's.

the explanation provided

You can assign roles at the table level allowing each table to reside in the same dataset.  

Can someone explain a little bit more on this? why D is wrong?

thanks a lot!