What does the side-by-side extensibility option mean?

What does the side-by-side extensibility option mean?
A . Extensibility based on the SAP cloud platform and published APIs
B. Transfer reporting to BW/4HANA
C. Context-aware SAP S/4HANA extensions, focused on tight integration
D. Have two SAP S/4HANA instances-one for standard code, and another for customer developments

Answer: A