For measuring the rate of transpiration of a twig using Ganong's potometer, learner A cuts the twig and fits it in the broad end of the water-filled potometer. Learner B cuts the twig under water and fits in the broad end of the potometer. Both the learners introduce an air bubble in the stem of the potometer, make the apparatus air-tight and start taking readings. What would they observe?

a.  Both A and B observe air bubble moving towards the twig.

b.  Both A and B observe no movement of air bubble.

c.   A observes no movement and B observes movement of air bubble towards the twig.

d.  A observes movement, while B observes no movement of air bubble towards the twig.