Suppose a table contains, among others, the following columns StdNo, Section, Grade and it is known that both StdNo,Section,Grade and StdNo,Section are superkeys but that neither StdNo nor Section is a superkey. Which ONE of the following is TRUE?

Select one:

a. Either StdNo or Section could be chosen as the primary key.

b. StdNo,Section could be chosen as the primary key but not a as candidate key.

c. StdNo,Section,Grade could be chosen as the primary key.

d. StdNo,Section is a candidate key and could serve as the primary key.

e. StdNo,Section,Grade is a candidate key.