A fair $6$-sided die is rolled once. What is the probability that the number on the top is an odd number? (adapted from 2011 AMC 8 Problem, Question \#18) 
选项为:A:$\frac16$  B:$\frac13$  C:$\frac12$  D:$\frac23$  E:$\frac56$  请回答其中一个选项.