Answer
The light reactions need ADP and $NADP^{+}$ to proceed, so that they can make ATP and NADPH. These are available after the ATP and NADPH have been used up by the Calvin cycle, so the light reactions cannot keep running unless the Calvin cycle sends the appropriate materials back to the light reactions.
Work Step by Step
This question is asking you to think in reverse since we usually think of the light reactions supporting the Calvin cycle, but here you need to think about how the reverse is true and materials from the Calvin cycle support the light reactions.
