MS Excel
Comments can be added to cells using

Insert > Comments
View > Comments
File > Comments
Edit > Comments

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MS Excel
You can select a single range of cells by

Clicking the upper-left cell in a group of cells and then pressing the Shift key while clicking the lower right cell in a group of cells
Dragging over the desired cells
Pressing the Shift key and an arrow key
Pressing the Ctrl key while dragging over the desired cells

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