Let's assume the price of a citron is "x" units and the price of a fragrant wood apple is "y" units.
From the given information, we can form two equations:
7x + 9y = 135 ...(1)
9x + 7y = 137 ...(2)
We can solve this system of equations using the elimination method. Multiplying equation (1) by 9 and equation (2) by 7, we get:
63x + 81y = 1215 ...(3)
63x + 49y = 959 ...(4)
Subtracting equation (4) from equation (3), we get:
32y = 256
y = 8
Substituting the value of y in equation (1), we get:
7x + 9(8) = 135
7x + 72 = 135
7x = 63
x = 9
Therefore, the price of a citron is 9 units and the price of a fragrant wood apple is 8 units.