Before acquiring a large area image, it is often desirable to quickly optimize the scan parameters $($e$.$g$.,$ Feedback Gain, PeakForce Amplitude, etc.$)$ while imaging a smaller area. Choose which parameter$($s$)$ should be held constant between the two images for this method to work. Explain your reasoning.

$($a$)$ Scan Rate

$($b$)$ Tip Velocity

$($c$)$ Samples$/$Line