A postman was returning to the post office which was in front of him to the north. When the post office was 100 m away from him, he turned left and moved 50 m to deliver the last letter at the Shanti villa. He, then moved in the same direction for 40 m, turned to his right and moved 100 m. How many metres was he away from the post office?

A

90 m

B

150 m

C

10 m

D

100 m