how much each bulb weighs how long each bulb lasts how much heat each bulb releases how much metal each bulb contains
I gave my students this multiple choice question: A light bulb manufacturer claims its light bulb is "twice as bright" per unit of electrical energy consumed as the leading light bulb. If this claim is correct, what factor must be accurately compared between the two bulbs?

Only one answer is correct among these 4 choices:
- how much each bulb weighs
- how long each bulb lasts
- how much heat each bulb releases
- how much metal each bulb contains

Could you tell me which one is correct?

how much heat each bulb releases