Which isotope is used to study photosynthesis?

$T$. $W$. Engelmann performed experiments on $.......$.

The carbon isotope used for the study of photosynthesis is ........

If a photosynthesising plant releases oxygen containing more than the normal amount of $^{18}O$,it is concluded that the plant has been supplied with:

The law of limiting factors in photosynthesis was proposed by Blackman in $1905$. Which of the following scientists criticised this law?

What is the by-product of bacterial photosynthesis?