Alpha rays emitted from a radioactive substance are

Out of the following,choose the ray which does not travel with the velocity of light.

If $_a^b X$ emits a positron,then two $\alpha$-particles,two $\beta$-particles,and finally one $\alpha$-particle,it transforms into $_d^c Y$. What is the correct relationship?

During the radioactive decay of a substance,which of the following is $NOT$ emitted?

An atomic nucleus $_{90}Th^{232}$ emits several $\alpha$ and $\beta$-radiations and finally reduces to $_{82}Pb^{208}$. It must have emitted:

The '$rad$' is the correct unit used to report the measurement of