Consider the following:
"There are 4 possible choices (A, B, C, D) for each question. Write down the option which you consider correct."

and
"There are 4 possible choices (A, B, C, D) for each question. Only one option is correct. Choose the correct option and write it down."

Do you know that teachers often give the instructions as according to the first one rather than the second one? The reason is because this allows them room for argument so they don't have to nullify the question. This is quite unfair because your elimination options won't work. For example, let's have a question...

What is 3+4?
A) 7
B) Seven
C) VII
D) 20

Obviously the answer is 7. But instincts tell you that A = B = C so if A is correct, B and C must be wrong, which contradicts what you think. So because of this, you put D because it's the odd one out.

So what happens is that the teachers realise there is a mistake in the options. But it is too late to change the options. So they changed the answer key such that those who answered either A, B or C get 1 mark while those who answered D scored 0!


On the other hand, if let's say you come late for a 1-hour MCQ exam by 59 minutes, you rush into the examination hall and grab a pencil for shading the optical marksheet, shade all your answers as B. The reason is that for exams which the setters don't know how to set options, the answers will mostly be either B or C. However, if you shade C, what if the whole paper is a true/false questionnaire? You get all the answers wrong.