Two wires A and B are of lengths 40 cm and 30 cm. A is bent into a circle of radius r and B into an arc of radius r. A current $t_1$ is passed through A and $t_2$ through B. To have the same magnetic induction at the center, the ratio of $t_1:t_2$ is

Question

\[\begin {array} {1 1} (a)\;3:4 & \quad (b)\;3:5 \\ (c)\;2:3 & \quad (d)\;4:3 \end {array}\]