The bar graph shows the grades in a mathematics class for the last grading period. If A, B, C and D are satisfactory grades, what fraction of the grades shown in the graph are satisfactory?
Answer Choices
A. \dfrac{1}{2}
B. \dfrac{2}{3}
C. \dfrac{3}{4}
D. \dfrac{4}{5}
E. \dfrac{9}{10}