The graph shows the forecast wind speed (in \mathrm{km} / \mathrm{h} ) during a 7-day period. Jack can sail alone only when the forecast wind speed is less than 20 \mathrm{~km} / \mathrm{h}. During this 7-day period, on how many days will Jack be able to sail alone?
Answer Choices
A. 4
B. 6
C. 1
D. 2
E. 5