Emmerson defined 'red drop' as a decline in photosynthesis,also termed the Emmerson effect. It appears at:

Which of the following occurs first in the light reaction?

Assertion $(A) : H^+$ gradient across the thylakoid membrane is essential for the formation of $\text{ATP}$.
Reason $(R) :$ Breakdown of $H^+$ gradient due to the movement of protons through $F_0$ of $\text{ATP}$ synthase releases enough energy to activate $\text{ATP}$ synthase enzyme that catalyses the formation of $\text{ATP}$.

Photosynthesis is an oxidation-reduction process. In this process,the substance that gets oxidized is ......... .

In photosystem $II$, the reaction centre chlorophyll-a absorbs $A$ nm wavelength of red light, causing electrons to become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked up by an $B$, which passes them to an electron transport system consisting of $C$. Pick the right choice for $A, B$ and $C$.

What are the products of the Hill reaction?