Hello, i'm currently doing some economics exercice for a futur assignement...

And i struggle with a question:

Which of the following events would contribute to economies of scale?

Select one or more:

a.Large supermarkets being able to drive down the cost of purchasing milk as it increases its scale of business.

b.The higher the number of users of Windows 10, the more likely it is that Microsoft will be able to achieve a higher sale of its operating system.

c.The expansion of the workforce requiring extra layers of management for better performance monitoring.

d.A merger of two firms leading to savings on the costs of HR, IT, and legal departments.

For me only the last answer is right but for my relatives, all answers can be right and i don't know who to believe...

Could you help me?

thanks very much