physics question electricty

Hi, please could I have some help on this question? I understand why B and D are correct but I don't understand why A is incorrect? I thought C should be incorrect because P=I^2R and since resistance is increasing, the power dissipated should increase from the fixed resistor?
question: https://ibb.co/PWnQxYf
thanks!

Option A is incorrect because the total resistance does not double (total resistance increases from 10.5 Ω to 20.5 Ω and 10.5 is NOT half of 20.5).
To show whether option C is correct or incorrect, we cannot use P = I²R as both current and resistance vary like current decreases and resistance increases.
Try to find the power dissipated in terms of emf and resistance and you should be able to determine if option C is good or bad.

oh yes, I forgot to take into account the resistance of the fixed resistor. Regarding this "Try to find the power dissipated in terms of emf and resistance" I can't think of an equation that links these, please could you give another hint? thanks!

We know the current is emf divided by total resistance and how can this link to P = I²R.

P=E^2/R?

Close but not really correct for this case.
I mention total resistance and R is not total resistance.

E=I*resistnace of internal resistor so E=0.5I so if I sub this in P=I2R then P=(E/0.5)^2/R?