# Some radio waves of frequency of about $1.5 \times 10^9$ Hz were received by a radio-telescope from distant star. If the speed of the waves is $3 \times 10^5 \: km/s$, then the wave- length of the wave will be:
$\begin {array} {1 1} (a)\;0.1 \: m & \quad (b)\;0.6\: m \\ (c)\;0.2\: m & \quad (d)\;0.46\: m \end {array}$