To have the drop-down permanently displayed, you need to use a Combo box. There are 2 kinds -- one is found on the Forms toolbar (View | Toolbars | Forms). Use this one, and follow these instructions:
1. Drag a Combo box onto the spreadsheet -- don't worry where at this point. Once it's there, use the handles to resize it so you get a list area with a button at the right. If you just click on the forms toolbox and click on the worksheet, you will likely get a square with only a drop-down arrow. Just resize...
Note -- if you click outside the control at any point, you activate it. To go back to editing mode, right-click on it and Format Control.
2. On the Control tab are 2 input boxes. Input Range is for the list that will populate the combo. Click the range filder button, select the input cells, and hit Enter. Cell Link is the cell where you will store the output. Click the range finder again, select the output cell, and click Enter twice.
Try it out -- if it works, re-enter Edit mode (as above) and reposition / resize the control so it's over the output cell.
That's it. Trouble is, if you want to have this for 20 cells, it's a big job.
Denis