Which should be my independent variable for my Fruit Bulb science fair project?

I'm doing an experiment where I'm using citrus fruit to light a tiny light bulb, like a Christmas tree light. I can only use one thing as my independent variable, which is what I'm changing in my experiment. Should I change the types of fruit, like an orange, lemon, and lime, or keep all the same types of fruit. Or, should I keep the same type of fruit and change the weight. Or, should I just change the type of light bulb?Which should be my independent variable for my Fruit Bulb science fair project?
I would change the fruit, to test the acidity to then see which one is the best conductor of electricity.