It depends, because the two options of this question just cover all the possibilities, because Ia+bI can't be equal to Ia-bI, otherwise b=0 and ab is less than 0, so it's impossible, so these two options cover all the possibilities and there must be a correct answer. In other words, as long as two or more options cover all the possibilities, one of them must be the correct answer. This is the law I have summed up. In fact, I usually like to use this rule. This is also a problem-solving skill, which can save time and get high marks. I personally like solving math problems. You can ask me if you don't understand. I'm not as boring as others who just answer questions for Baidu to know the value of wealth. Nobody answered your question. It must be because your reward score is 0. I don't care about the reward points. I just like solving math problems. Ask me if you have any questions. It doesn't matter whether there are reward points or not.