How to Merge unallocated space into one partition
Before merging unallocated space you should:
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How to Merge unallocated space into one partition (see How to Merge two partitions into one):
Step1. Right-click C drive and select “Merge Partitions”

Step2. In the pop-up window, tick the checkbox of unallocated space, and then click “OK” to finish.

Step3. Click “Apply” on the toolbar to execute pending operations.

Enjoy!
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Source: extend-partition