For the first 30 minutes of a trip, Deanna drove at a constant speed. For the next 30 minutes, she drove at a constant speed 20\:km/h faster than her original speed. If the total distance that she travelled was 100\:km, how fast did she drive for the first 30 minutes?
Answer Choices
A. 80\:km/h
B. 90\:km/h
C. 100\:km/h
D. 110\:km/h
E. 120\:km/h