Hi VichitraVir
Great question! The answer actually applies across the GMAT (both quant and verbal).
When you reach answer C and you feel it correctly answers the question, it's important to realize you "think it's correct". That's based on all the analysis, trap checking etc that you've done so far. But what if you've missed something. Answers D and E can sometimes trigger that realization -- that there's a trap or angle that you've missed.
Remember, the GMAT is test of reasoning. So your job is to choose the BEST answer out of 5. How can you do that if you don't read all 5?
The GMAT test writers know that you might stop at C if you think it's reasonable. That's a potential trap on its own.
So read all 5 before choosing the best one!
Best of luck!
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