Same quantity of ice is filled in each of the two metal containers P and Q having the same size, shape and wall thickness but made of different materials. The containers are kept in identical surroundings. The ice in P melts completely in time `t_1` whereas that in Q takes a time `t_2`. The ratio of thermal conductivities of the materials of P and Q is

Options

A. `t_2 : t_1`
B. `t_1 : t_2`
C. `t_1^2 : t_2^2`
D. `t_2^2 : t_1^2`