${ }_{35}{\mathrm{X}}^{88}$ is an unstable isotope, decays in two successive steps to produce stable(s) isotope ${ }_{36}{\mathrm{Y}}^{87}$ ,as shown as ${}_{35}X^{88} \stackrel{\mathrm{I}}{\to }\mathrm{Z}{\stackrel{\mathrm{II}}{\to }}_{36}{\mathrm{Y}}^{87}$

The correct statement(s) is/are 

• A

  I involve a $\mathrm{\beta }$-emission

• B

  II involve a neutron emission

• C

  If I involve $\mathrm{\beta }$-emission, y and Z must be isotopes

• D

  X and Z will always be isotones