A fair $6$-sided die is rolled twice. What is the probability that the first number that comes up is greater than or equal to the second number? (2011 AMC 8 Problems, Question \#18) 
选项为:A:$\dfrac{1}{6}$  B:$\dfrac{5}{12}$  C:$\dfrac{1}{2}$  D:$\dfrac{7}{12}$  E:$\dfrac{5}{6}$  请回答其中一个选项.