Avg age of employees

Average age of employees working in a department is 30 years. In the next year, ten workers will retire. 
What will be the average age in the next year?

I. Retirement age is 60 years.

II. There are 50 employees in the department.

A. I alone sufficient while II alone not sufficient to answer
B. II alone sufficient while I alone not sufficient to answer
C. Either I or II alone sufficient to answer
D. Both I and II are not sufficient to answer
E. Both I and II are necessary to answer

Watch this space for answer in a week.