[单选题]
The expression (MX - M b1fuj /74qdrb. kkmt*qlg )xzaia7rnwim,f paob .k , :(07c(Y - Ma) 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.