Please choose the correct answer to the 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?

Answer1: how much each bulb weighs Answer2: how long each bulb lasts Answer3: how much heat each bulb releases Answer4: how much metal each bulb contains

Answer format: answer1/answer2/answer3/answer4
Answer: answer3