PopWatch HeadScratcher No. 39

Another PopWatch HeadScratcher to get you through the weekend:

We’re going to try something a little different this week. Consider, if you will, the following five bands:

The Commodores
Led Zeppelin
The Spin Doctors
U2
The Wallflowers

Now, put them in an order — and be prepared to explain the logic behind said order. But there’s a catch: There’s a gap in the sequence. Tell us why the gap is there. [Bonus Points: Name a band that could fill the gap – there's more than one possible answer.]

Send your best guess to popwatch@ew.com. Be sure to write "HeadScratcher" in the subject line and include your first and last names in your answer. I’ll post the most interesting responses Monday afternoon.