In relation to employment law:

(a) Explain the circumstances in which it may be appropriate for an employer to enter into a fixed term labour contract with an employee. (4 marks)

数学学习中由数字运算到字母运算的转化，属于( )。

A．自上而下的迁移

B．自下而上的迁移

C．正迁移

D．负迁移

E．水平迁移