Fill in with a suitable conjunction.

Mahi studied very hard _______ she got low marks in the exam.

Answer: D. None of the above

The correct answer is D. None of the above.
The sentence shows a contrast between studying hard and getting low marks.
The most natural conjunctions to use here would be but, yet, or however (e.g., "...hard, but she got less marks").
"Then also" (A) and "also" (B) are grammatically incorrect in this context.
"But also" (C) is only used as part of the pair "not only... but also

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