MS Excel
The command Edit > Fill Across Worksheet is active only when

When no sheet is selected
None of the listed here
When many sheets are selected
One sheet is selected

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MS Excel
You can select a single range of cells by

Pressing the Ctrl key while dragging over the desired cells
Dragging over the desired cells
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
Pressing the Shift key and an arrow key

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