Answer:
A. set_union
Explanation:
The algorithm set_union is used to find the elements in one range of elements that do not appear in another range of elements.
<span>An attribute in a relation of a database that serves as the primary key of another relation in the same database is called a</span>
<span>If a user name can have seven letters (with no distinction between upper and lower case), and if a letter can be repeated, then the maximum number of names is 26x26x26x26x26x26x26, or 8,031,810,176. That is, slightly more than eight billion names are possible.</span>
Lookup the isalpha() function in the C standard library, this function will check and return non-zero for alphabetic characters.