设s，t为两个命题，对于“st”的逻辑涵义，下面叙述中：　　a)如果s，则t 　　b)s当且仅当t 　　c)r与s互为充分条件　　d)s仅当t 　正确的是

A．a select statement that selects a subset of fields in a table
B．a select statement that returns a subset of the data in a table
C．a select statement that appears within another select statement
D．a select statement that returns a subset of the constraints on a field

A．辅存
B．虚拟存储
C．外存
D．内存