Which of the following are unchanged by a chemical reaction?

May someone explain how to do this question?

Like a key in a lock, the shape of the. must bind with the. of the recieving neuron

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.