Which of the following pairs consists of molecules having the same mass number?

$\mathrm{In} {\mathrm{D}}_2\mathrm{O}$

$\left({}_1\mathrm{H}_2^2\mathrm{O}\right)$ Total mass number of two ${}_1{}^2\mathrm{H}$ isotopes is 4

In HTO $\left({}_1\mathrm{H}^1{}_1\mathrm{H}^3\mathrm{O}\right)$ Total mass number ${}_1{}^1\mathrm{H} \mathrm{and} {}_1\mathrm{H}^3$isotopes is 4.

${\mathrm{D}}_2\mathrm{O}$=1+16=20

HTO=1+3+16=20