[单选题]
The expression (MX - MY1zpo.x+m1)y-jobd yf ;1bqsa - Mwfnp x+f(fj zk( )a+k+a) x 931.5 represents
A. the binding energy of the nucleus X.
B. the binding energy of the nucleus Y.
C. the energy released when nucleus X undergoes alpha decay.
D. the energy released when nucleus Y undergoes alpha decay.