question_answer

The graph below shows the result of   experiment in which isotopes of oxygen were used to distinguish between oxygen absorbed by plants and oxygen given out. A mixture of the oxygen istopes \[^{16}{{O}_{2}}\] and \[^{18}{{O}_{2}}\] was supplied to a suspension of the unicellular alga Chlorella which had previously been exposed to \[^{16}{{O}_{2}}\] only. During the following hour changes in the concentration of these gases in the suspension were measured. The \[^{18}{{O}_{2}}\] concentration fell in light because it was:                    

A) Undergoing radioactive decay to from \[^{16}{{O}_{2}}\]

B) Absorbed in respiration but was not being produced in photosynthesis.

C) Absorbed in respiration slower than it was being produced in photosynthesis.

D) Absorbed as C\[^{18}{{O}_{2}}\] in photosynthesis.