Hey everyone. I realize that we assumed 1/2 and -1/2 for z and found that only option A stands valid, but when I used -1 and 1 for z, I see that both A and C are right.
Where am I going wrong?
You are absolutely correct.
But the problem is, the Question states that, choose the option which must be correct.
So, you need to consider the case which must always be true.
If the question would have stated that, choose the answer which could be true, then options A and C can be chosen as a correct answers.
But due to the word must. Only option A is correct.
Thank you!
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