During α-emission :
(a) the mass number and atomic number of an atom decrease by 2 a mu.
(b) the mass number decreases by 4 amu and atomic number decreases by 2 amu.
(c) the mass number *** unchanged, but atomic number decreases by 2 amu.
(d) the mass number decreases by 4 amu and atomic number *** unchanged.
