旅游地图属于（）。

A. 油浸式调压器
B. 自耦式调压器
C. 感应调压器
D. 隔离调压器

A subquery can retrieve zero or more rows
B. Only two subqueries can be placed atone level
C. A subquery can be used only in SQL query statements
D. A subquery can appear on either side of a comparison operator
E. There is no limit on the number of subquery levels in the WHERE clause of a SELECT statement&e

A. 1
B. 4
C. 10
D. 20