[单选题]
The binding energy per np6 ryc7.fj( qbng s-hzr-ctyx34+ m4vucleonj5( k2 *ubxits 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. $\frac{E}{Zc^2}$
