Suppose you display a 1 khz, 2 v sine wave in channel a and a dc, 1v signal in channel b with sensitivities set on 1v/div. you select 'add' so that the two signals are combined and you readjust the position so the trace is in the middle of the screen. if you switch the signal in channel b from dc to ac, what will happen? (draw a picture if it .)

You are going to see the AC functions shifted down

Explanation:

Because you had a positive DC signal first, you had to readjust the position to the middle, otherwise, the sum of AC and DC would have been shifted up; because of this, your cursor has been moved down and ones the DC disappears from channel B the signal will be shifted down.

Note: Changing from DC to AC in channel B will kill all the DC signal