Write the sum or difference in simplest form.
On Monday, Cathy read start fraction 1 over 6 end fraction of her book, and on Tuesday, she read start fraction 2 over 6 end fraction of the book. How much has she read?
A. start fraction 1 over 6 end fraction
B. one-fifth
C. start fraction 1 over 3 end fraction
D. one-half