If an individual proton has mass 1.007825 amu and an individual neutron has mass 1.008665 amu, and the observed atomic mass of gadolinium-157 is 156.92396 amu, what's its mass defect?
Question options:

A)

–1.38269 amu

B)

–1.35833 amu

C)

1.35833 amu

D)

1.38269 amu