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【单选题】
下列的SQL语句中，哪一个语句不是数据定义语句A.
CREATE INDEX
B.
DROP VIEW
C.
REVOKE
D.
ALTER TABLE
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【简答题】Photoshop中，图层样式中的_____________可以制作出图层的立体效果。
【单选题】A worldwide ban on the production of certain ozone-destroying chemicals would provide only an illusion of protection. Quantities of such chemicals, already produced, exist as coolants in millions of r...
A.
It is impossible to measure with accuracy the quantity of ozone-destroying chemicals that exist as coolants in refrigerators.
B.
In modern societies, refrigeration of food is necessary to prevent unhealthy and potentially life-threatening conditions.
C.
Replacement chemicals that will not destroy ozone have not yet been developed and would be more expensive than the chemicals now used as coolants in refrigerators.
D.
Even if people should give up the use of refrigeration, the coolants already in existing refrigerators are a threat to atmospheric ozone.
E.
The coolants in refrigerators can be fully recovered at the end of the useful life of the refrigerators and reused.