Explain why:
$(a)$ To keep a piece of paper horizontal,you should blow over,not under,it.
$(b)$ When we try to close a water tap with our fingers,fast jets of water gush through the openings between our fingers.
$(c)$ The size of the needle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection.
$(d)$ $A$ fluid flowing out of a small hole in a vessel results in a backward thrust on the vessel.

(N/A) According to Bernoulli's principle,as the velocity of a fluid increases,its pressure decreases. Blowing over the paper increases the air velocity above it,reducing the pressure above compared to below,which creates an upward lift to keep the paper horizontal.
$(b)$ According to the equation of continuity,$A_1v_1 = A_2v_2$ (Area $\times$ Velocity = Constant). When we partially close the tap with our fingers,the cross-sectional area of the opening decreases,causing the velocity of the water to increase significantly,resulting in fast jets.
$(c)$ The flow rate is governed by the equation of continuity. The needle has a very small cross-sectional area compared to the syringe barrel. Even a small change in the needle's diameter significantly affects the flow velocity,making it a more dominant factor in controlling the flow rate than the thumb pressure.
$(d)$ As fluid exits the hole with high velocity,it carries momentum. According to the law of conservation of momentum,to keep the total momentum of the system zero,the vessel must experience an equal and opposite momentum,resulting in a backward thrust.