Greta added 3/6 of a cup of sugar to her cake mix then sprinkled 1/6 of a cup of sugar over the top of the finished which fraction represents the total amount of sugar that Greta used for the cake? Express the answer in the lowest terms?
a: 1/6 of a cup
b: 1/3 of a cup
c: 3/6 of a cup
d: 2/3 of a cup