影响刀具的锋利程度、减小切削变形、减小切削力的刀具角度是（）。
A. 主偏角
B. 前角
C. 副偏角
D. 刃倾角
E. 后角
Which two statements are true regarding views?（）
A subquery that defines a view cannot include the GROUP BY clause
B. A view that is created with the subquery having the DISTINCT keyword can be updated
C. A view that is created with the subquery having the pseudo column ROWNUM keyword cannot be updated
D. A data manipulation language (DML) operation can be performed on a view that is created with the subquery having all the NOT NULL columns of a table
同学们喜欢的NBA球星中有许多都是黑人，他们的祖先来自非洲大陆。历史上与非洲黑人大批来到北美有关的史实是（）
A. 三角贸易
B. 独立战争
C. 南北战争
D. 非洲独立年
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