At the start of a session, the number of students in grades 6, 7 and 8 in a school was 120, 150 and 130 respectively.
Later on 10 students were admitted in grades 6 and 7 and 10 students were transferred from grade 8. The ratio of the number of students at the start and those at the end of the session, provided no further admission or withdrawal/transfer had taken place, was:

A

20 : 21

B

40 : 41

C

3 : 5

D

13 : 42