Each Row of the Table Department Describes a Department

$Table~Employee\\\begin{array}{|l|l|l|l|l|}\hline \text { Empno } & \text { Empname } & \text { Empdeptno } & \text { Empsalary } & \text { Emptitle } \\\hline\end{array}$
Each row of the table Department describes a department. Deptno is the primary key. Deptmgr (Department manager) refers to Empno in table Employee.

$Table~Department\\\begin{array}{|l|l|l|l|}\hline \text { Deptno } & \text { Deptname } & \text { Deptcity } & \text { Deptmgr } \\\hline\end{array}$
Each row of the table Project describes a project. The primary key is Projno. Projmgr (project manager. refers to Empno in the table Employee.

$Table~Project\\\begin{array}{|l|l|l|}\hline \text { Projno } & \text { Projname } & \text { Projmgr } \\\hline\end{array}$
The table Projstaff indicates the staff assigned to various Projects.

$Table~Project\\\begin{array}{|l|l|l|l|l|}\hline \text { Prstno } & \text { Prstempno } & \text { Prsthours } & \text { Prstdate } & \text { Prstasgtno } \\\hline\end{array}$
The combination of columns Prstno, Prstempno is a candidate key. There could be other candidate
keys. Prstempno is the Empno in table Employee of the employee working on project number Prstno, which refers to Projno in the table Project. The meaning of Prstasgtno is left undefined.
-Which of the following columns or combination of columns may not be a candidate key of the table Projstaff?

A) Prstno, Prstempno
B) Prstasgtno
C) Prstno, Prstempno, Prsthours
D) Prstno, Prstasgtno