What is cloud computing?

a$.$ A computing model that provides ubiquitous, on$-$demand access to a shared pool of configurable resources that can be rapidly provisioned

b$.$ A computing model that provides ubiquitous, shared access to a shared pool of configurable resources that can be rapidly provisioned

c$.$ A computing model that provides ubiquitous, shared access to a shared pool of configurable resources

d$.$ A computing model that provides ubiquitous, on$-$demand access to a shared pool of configurable resources