Please help!:
Isabella wants to buy shirts for her friends who wear small, medium, and large sizes. She has money to buy only one shirt.To help her decide which shirt to buy, she uses a spinner that has been divided into three equal sections and has the three sizes written on each section. What is the probability that she will buy a medium-sized shirt?
A. 1/4
B. 2/3
C. 1/3
D. 1