Which one of the following pairs represents isobars?

Match the following atomic species with their correct classification:

$(i) \ { }_{26} Fe ^{54}, { }_{26} Fe ^{56}, { }_{26} Fe ^{57}, { }_{26} Fe ^{58}$	$(a)$ Isotopes
$(ii) \ { }_1 H ^3, { }_2 He ^3$	$(b)$ Isotones
$(iii) \ { }_{32} Ge ^{76}, { }_{33} As ^{77}$	$(c)$ Isodiaphers
$(iv) \ { }_{92} U ^{235}, { }_{90} Th ^{231}$	$(d)$ Isobars
$(v) \ { }_1 H ^1, { }_1 D ^2, { }_1 T^3$

Which one of the following is not an iso-electronic pair?

In the two elements,${ }_{Z_1} A^{M_1}$ and ${ }_{Z_2} B^{M_2}$,the following relations are true. $M_1 \neq M_2$ and $Z_1 \neq Z_2$ but $M_1-Z_1=M_2-Z_2$. These elements are ($M$ is atomic weight,$Z$ is atomic number)

On comparing the chemical reactivity of $C^{12}$ and $C^{14}$,it is revealed that:

To determine the masses of the isotopes of an element,which of the following techniques is useful?