Which statement is true regarding the rate of transpiration in a dorsiventral (dicot) leaf?

$A$ : During stomatal opening there is relative change in $TP$ of guard cell and subsidiary cell.
$R$ : $TP$ of subsidiary cell decreases during opening and that of guard cells increase.

$A.$ In dry atmosphere,the relative humidity is low,so the rate of transpiration increases.
$B.$ Slow breeze promotes the rate of transpiration.
$C.$ $ABA$ promotes transpiration.
$D.$ $A$ high salt concentration in soil water increases transpiration.

The ultimate force which propels water against gravity is generated by

Stomatal index $(I)$ is equal to:

Water moves up against gravity and even for a tree of $20 \,m$ height, the tip receives water within two hours. The most important physiological phenomenon which is responsible for the upward movement of water is $...........$