A man covers a certain distances between his house and office on scooter. With an average speed of 30 km/hr, he is late by 10 min., however, with a speed 40 km/hr, he reaches his office 5 min earlier. Find the distance between his house and the office.

• A.  20 km    
• B.  10 km
• C.  15 km    
• D.  30 km