A nucleus of mass $M + \Delta m$ is at rest and decays into two daughter nuclei of equal mass $\frac { M } { 2 }$ each. Speed of light is $C$. The speed of daughter nuclei is\\
(1) $c \frac { \Delta m } { M + \Delta m }$\\
(2) $c \sqrt { \frac { 2 \Delta m } { M } }$\\
(3) $c \sqrt { \frac { \Delta \mathrm {~m} } { \mathrm { M } } }$\\
(4) $c \sqrt { \frac { \Delta m } { M + \Delta m } }$