How does this potential divider circuit work?

[Dr. Who]

Homework Statement

Below is an MCQ that appeared in a GCSE exam. I am confused about the solution. Please guide.

Relevant Equations

I= V/R (Ohm's Law)

The correct option is supposed to be (B). Can someone please explain why changing the resistance of the variable resistor affects the voltage across the other resistor? Also explain why (B) is the correct option? Thanks

 

[phinds]

changing the resistance changes the current through the circuit, which in turn changes the voltages across the resistors. The TOTAL voltage across the two can't change but the ratio clearly changes. Think in extremes. What happens if the variable resistor is taken all the way to zero?