[单选题]
The binding energy per nuclezv a808 ce8tyc4b8t ; bh,tno:2eejyf ah)ws-utf3h6on of a $^A_ZX$ nucleus is E. What is the mass defect for the formation of X from its constituent nucleons?
A. $\frac{AE}{c^2}$
B. $\frac{ZE}{c^2}$
C. $\frac{E}{Ac^2}$
D. $\frac{E}{Zc^2}$
