$A$ 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 to the left and moved $50 \, m$ to deliver the last letter at Shanti Villa. He then moved in the same direction for $40 \, m$,turned to his right,and moved $100 \, m$. How many metres away was he now from the Post Office? (in $m$)

• A
  $0$
• B
  $150$
• C
  $90$
• D
  $100$