What is believed to be the second most common element in the universe?

Isotope$(s)$ of hydrogen which emits low energy $\beta^{-}$ particle with $t_{1/2}$ value $> 12 \ years$ is/are :

Match the following types of hydrides in List-$I$ with their examples in List-$II$.

List-$I$ (Type of hydride)	List-$II$ (Example)
$(A)$ Electron precise	$(i)$ $SiH_4$
$(B)$ Saline	$(ii)$ $H_2O$
$(C)$ Electron deficient	$(iii)$ $MgH_2$
$(D)$ Electron rich	$(iv)$ $B_2H_6$

The hydride that is $NOT$ electron deficient is

Which of the following is $NOT$ true regarding the usage of hydrogen as a fuel?

Which property among the following is the same for both hydrogen and deuterium molecules?