c
(c) According to Newton's law of cooling
Rate of cooling ? mean temperature difference.
Initially, mean temperature difference $ = \left( {\frac{{70 + 60}}{2} - {\theta _0}} \right) = (65 - {\theta _0})$
Finally, mean temperature difference $ = \left( {\frac{{60 + 50}}{2} - {\theta _0}} \right) = (55 - {\theta _0})$
In second case mean temperature difference decreases, so rate of fall of temperature decreases, so it takes more time to cool through the same range.