Which one of the following is most acidic?

The oxygen evolved during photosynthesis comes from water molecules. Which one of the following pairs of elements is involved in this reaction?

$\lim _{x \rightarrow 0} \frac{\tan ^3 x-\sin ^3 x}{x^5}$ is equal to

Cornea transplant in humans is almost never rejected. This is because

The binding energy of deuteron ${}_1^2H$ is $1.112 \, MeV$ per nucleon and an $\alpha$-particle ${}_2^4He$ has a binding energy of $7.047 \, MeV$ per nucleon. Then in the fusion reaction ${}_1^2H + {}_1^2H \to {}_2^4He + Q$,the energy $Q$ released is ......... $MeV$.

The number and type of bonds between $2$ carbon atoms in $CaC_2$ are: