What does it mean variable Direct Current ?

[vkash]

what does it mean "variable Direct Current"?

I am confused on the point where i see the word variable DC in a question(see attachment). If a DC vary like a curve then in some cases it can be like AC.
How is it varying
(Don't tell me answers.)

 

[SammyS]

I am confused on the point where i see the word variable DC in a question(see attachment). If a DC vary like a curve then in some cases it can be like AC.
How is it varying
(Don't tell me answers.)

The DC voltage source is not varying , it's variable .

You apply a DC voltage at some value, and measure V₁, V₂ and I.

Then, change the DC voltage to a different value, and measure V₁, V₂ and I.

   ⋮