If your working income was low, you may have been entitled to the Canada Workers Benefit. However, if you are in school full time, you are not eligible for this refundable credit. It is common for the Canada Revenue Agency (CRA) to make adjustments during the assessment process. This will be explained when you receive your Notice of Assessment.
As there are many reasons your refund may differ from your return sent, please see the Canada Revenue Agency's link to Understand your refund and contact them if needed.
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