Two spherical objects have equal masses and experience a gravitational force of 20 N towards one
another. Their centers are 30 cm apart.

  distance on the y-axis and time on the x-axis.

explanation:

the instantaneous velocity = change in position/time taken

the slope of the graph should give instantaneous velocity.

  slope of a graph = change in value of y -axis / change in values of x -axis

  comparing both the equations

    change in value of y -axis = change in position

  change in values of x -axis = time taken

so position values should be on the y axis and time values should be on the x axis.

distance on the y-axis and time on the x-axis.

option number c is the right answer.

the doppler effect is used by weather forecasters to measure 

moving clouds