Answer:
a. PGA down, RuBP up
Explanation:
RuBP, also known as Ribulose-1,5-bisphosphate is a molecule that assist in he fixation of carbon dioxide in the Calvin Cycle or light independent reaction of photosynthesis.
RuBP is combined with carbon dioxide using the enzyme known as rubisco to form an short-lived intermediate product which divides to form two 3-carbon molecule structure known as 3-phosphoglycerate.
If the concentration of carbon dioxide is suddenly reduced, it thus means that the rate of production of 3-phosphoglycerate will reduce as carbon dioxide becomes a limiting factor. Hence, 3-phosphoglycerate's  concentration will reduce while more RuBP will become available as a result.
The correct option is a.
