Help me which one is the correct one and why

Your target variable is δf, the magnitude of the difference in frequency between the waves emitted from the sonar device and the waves received by the device after reflecting off the whale. write an expression for δf in terms of the relevant frequencies using the subscript notation introduced above.

On a day when the barometer reads 75.23 cm, a reaction vessel holds 250 ml of ideal gas at 20 celsius. an oil manometer ( ρ= 810 kg/m^3) reads the pressure in the vessel to be 41 cm of oil and below atmospheric pressure. what volume will the gas occupy under s. t.p.?

Marcella is a four-year-old girl who is a victim of emotional child abuse. based on the child and family services improvement and innovation act of 2011, which action should the government take for marcella? a. the state should arrest marcella’s abuser. b. the state should find a home for marcella as soon as possible. c. the state should enforce stricter laws in the area where marcella resides. d. the state should provide medical assistance to marcella.