Two vessels of different materials are similar in size and other respects. The same quantity of ice filled in them gets melted in 40 and 70 minutes respectively. The ration of thermal conductivities for the materials of boxes is :

Options

A. `4:7`
B. `7:4`
C. `4:11`
D. None of these.

Answer

B

Explanation

`k=(Qd)/(A(T_(1)-T_(2))t)" "rArr" "kprop1/t`
`:.(k_(1))/(k_(2))=(t_(2))/(t_(1))=(70)/(40)`
`(k_(1))/(k_(2))=7/4`
Thus correct choice is (b).