Ella is preparing for a 20-mile endurance cycling competition. She practices by cycling at a constant speed of 12 miles per hour for the first 4 miles. She increases her speed by 1.5 miles per hour every mile until she reaches 12 miles. After crossing 12 miles, she reduces her speed by 3 miles per hour each mile, till she reaches a speed of 15 miles per hour and keeps it constant until the end of her ride.

From the graph shown below, find the interval where Ella's speed is increasing.

A.
Ella's speed is increasing between 12 and 15 miles.
B.
Ella's speed is increasing between 4 and 12 miles.
C.
Ella's speed is increasing between 16 and 18 miles.
D.
Ella's speed is increasing between 0 and 3 miles.