A basic premise of OGSA is that everything is (71) by a service: a network enabled entity that (72) some capability through the exchange of messages. Computational resources, storage resources, networks, programs, databases, and so forth are all (73) . This adoption of a uniform service-oriented model means that all components of the environment are virtual. More specifically, OGSA represents everything as a Grid service: a Web service that conforms (74) a set of conventions and supports standard interfaces for such purposes as lifetime management. This core set of consistent interfaces, from which all Grid services are implemented, (75) the construction of hierarchical, higher-order services that can be treated in a uniform way across layers of abstraction.
（72）处填（）。
A．provides
B．have
C．expands
D．limits
