Nathaniel and Fadi read books for Ms. Jackson class reading contest. Nathaniel read 9/28 of their class's total pages and Fadi read 3/28 of their class's total pages. What fraction of their class's total pages did Nathaniel and Fadi read in all? Simplify the answer if possible.
A. 3/56
B. 3/28
C. 3/14
D. 3/7