Maria's office is 1.5 km away from her house. She leaves her house at 8:30 am and walks at constant speed reaching her office 9:00 am. Along her way to the office is a grocery store which is located 1 km from her house. How many minutes would Maria spend walking from her house to the grocery store?
a. 10
b. 15
c. 20
d. 25
e. 30