Plz help me with a question

For a DC voltage, this only means the electric potential will not change direction but does not mean the magnitude is constant. So assuming no voltage applied to Y-axis and a DC voltage applied on the X-axis, there will be a dot on the oscilloscope if the magnitude is constant.

If the magnitude is not a constant, it will not be just a dot but a dot that moves along the X-axis. If the frequency is high enough, it will appear to be a line along the X-axis but only on either the positive side or the negative side.

For a sin voltage applied on the Y-axis (assuming no voltage applied to X-axis), the dot will be moving between + and - regent along the Y-axis. This will appears to be a line on the Y-axis if the dot moves fast enough (the frequency of the sin wave is high enough).

Combining both, if the voltage on X-axis is not a constant, the dot will be moving as a line sharped like "\" or "/" across either on the left side of Y-axis or the right side of Y-axis but not crossing the Y-axis. It will cross the X-axis.

If the voltage on X-axis is a constant, the dot will be moving along a straight line that will cross the X-axis but not the Y-axis.

The length of the locus of the dot on the Y-axis depends on the max and min value of the sin function. The length of the locus of the dot on the X-axis depends on the dc voltage applied (this could be a constant or a range). The slope of the line is the ratio of the max of length Y to the max of length of X.