The problem asks for the approximate difference between the temperature of maximum density of heavy water (D2O) and ordinary water (H2O). The temperature of maximum density of H2O is approximately 3.98∘C. The temperature of maximum density of D2O is approximately 11.2∘C. The difference in temperature in Celsius is ∆TC=11.2∘C−3.98∘C=7.22∘C The problem specifies the temperatures in Kelvin, with y being the temperature for H2O and x for D2O. The difference is (x−y) in Kelvin. The magnitude of a temperature difference is the same whether measured in Celsius or Kelvin. ∆TK=∆TC So, the difference in Kelvin is 7.22 K . The value 7.22 K is closest to the option 7.0 K .