Which of the following statements is true about data warehouse integration systems and virtual data integration systems?

a$.$A virtual data integration system uses ETL, or extract$-$transform$-$load, tool pipelines that periodically extract data from

the sources and load them an application.

b$.$Both data integration architectures use a mediated schema.

c$,$A data warehouse integration system leaves data in the sources and accesses them as needed at query time.

d$.$In the warehousing approach, the user poses queries in terms of the warehouse schema.