Bulliform cells respond to which of the following change?

Bulliform cells are large, bubble-shaped epidermal cells that occur in groups on the upper surface of the leaves of many grasses. Loss of turgor pressure in these cells causes leaves to roll up during water stress. During drought, the loss of moisture through vacuoles induces bulliform cells to cause the leaves of many grass species to close as the two edges of the grass blade fold up toward each other. Once adequate water is available, these cells enlarge and the leaves open again. So, bulliform cells respond to turgor pressure and not to osmotic pressure and photosensitisation. 
Thus, the correct answer is option A.