Answer: D
The ratio of the slant heights of two cones is \({l_1}:{l_2} = {5}:{7}\) \(\therefore\) The ratio of their curved surface areas is: \(= \pi r l_1 : \pi r l_2\) \(= l_1 : l_2\) \(= 5 : 7\)
The ratio of the slant heights of two cones is \({l_1}:{l_2} = {5}:{7}\) \(\therefore\) The ratio of their curved surface areas is: \(= \pi r l_1 : \pi r l_2\) \(= l_1 : l_2\) \(= 5 : 7\)