The average working hours per month of the staff aged over 50 yearsin a factory were 160 and that of the staff aged under 50 years were 210. The mean working hour per month of all the staff was 200. The ratio of the numberofthe staff aged over50 to thatof the staff aged under 50 is:

A

3 : 1

B

2 : 1

C

1 : 3

D

1 : 4