A small tree was planted at a height of 7 feet. The tree has been planted for 14 months, and is now 56 feet tall. Which equation could be used to find x, the average number of feet the tree grew each month?
A. 7x + 14 = 56
B. 14x + 7 = 56
C. 14x + 56 = 7
D. 56x + 7 = 14