Which defines the Global$-$and$-$Local$-$as$-$View $($GLAV$)$ schema mapping?

a$.$ Expressions include a query over the data sources on the left$-$hand side and a query on the mediated schema on the

right$-$hand side

b$.$Focuses on describing each data source as precisely as possible and independently of any other sources

c$.$ Defines the mediated schema as a set of views over the data sources

d $.$Derived from data dependency constraints and developed for the specification and analysis of integrity constraints in

databases