Which set of elements forms stable,covalently bonded hydrogen-bonded hydrides?

In a first order reaction,the concentration of the reactant decreases from $0.6 \ M$ to $0.3 \ M$ in $15 \ min$. The time taken for the concentration to change from $0.1 \ M$ to $0.025 \ M$ in minutes is

Which of the following pairs of orbitals possess two nodal planes?

Bell metal is an alloy of which of the following?

Statement $-1$ : $\sim (p \leftrightarrow \sim q)$ is equivalent to $p \leftrightarrow q$.
Statement $-2$ : $\sim (p \leftrightarrow \sim q)$ is a tautology.

$A$ compound $X$ with molecular formula $C_3H_8O$ can be oxidised to a compound $Y$ with the molecular formula $C_3H_6O_2$. $X$ is most likely to be